MG Sports Cars

On my 72 B-GT (still using MGB Master Cylinder), the brake pressure failure switch has now failed twice in last 15 years (failure is catastrophic - leaks brake fluid, which leads to complete system failure. Fortunately, I was in my driveway). I'm about to pull the switch and replace with a bronze screw or cap. Anybody else done this?

Hi Don,
When you say switch, do you mean the small plastic one, or the complete valve block?
If you are just replacing the plastic switch your leak will remain. The switch is not made to contain the brake fluid and in fact has a vent hole to let it out.
The leak is actually the valve block itself. Blocking the switch hole with a plug will compromise your brake system as the leak in the valve block worsens.
The proper repair is to replace the entire valve block. And probably never have that problem again.
Cheers
Fred

It is the "plastic portion" - when you say "valve block", are you talking about the manifold that sends brake fluid from the master cylinder to the 4 wheels?

Don, if you are talking about the "H" shaped pressure fail switch assembly it shouldn't have fluid access to the switch unless the internal seals and o-rings are worn. Moss has a rebuild kit available for those which should solve your problem and allow you to keep your warning switch. Even if you want to install a bolt in place of the switch you should rebuild the cylinder and shuttle valve, you don't want fluid from one side of the system going past the valve and being lost on the other side. Leaking seals also may be part of the reason you've had failures. The switch isn't designed to take hydraulic pressure against it.

Yep,

The "Manifold" contains a shuttle valve that turns the switch on in the event of a brake failure. This shuttle valve also cuts brake fluid flow to the affected circuit. When the valve is malfuntioning you run the risk of losing braking to the front or rear brakes. Or in your case losing all your brake fluid.
There should never be any fluid at the switch. If there is, then the seals in the valve block have failed and changing the switch portion will not fix the problem. Blocking the hole will result in a loss of braking eventually.
A kit is available to rebuild the valve block, but at nearly 40 years old chances of a good repairable part are slim.
I would put in a new one, flush the entire brake system with new fluid (do not use silicone fluid!), bleed it and your done.
Flush the system completely every two years regardless of mileage. Brake fluid is hygroscopic and will absorb moisture out of the air. This is what turns it black, and it then starts to attack your seals and corrode the metal parts.

Hope thats helpfull.
Cheers
Fred

I've had MG's for 33 years and I'm still learning - thanks, guys!

OK - Parts from Moss did not fit (they provided 4 o-rings to replace 2). And of course, you can't buy the whole assembly anymore. I'm about to throw out the valve block and "plumb" the front and back circuits separately from the master cylinder. Given that this switch has never worked and the circuit has failed "catatrophically" twice (loosing all brake fluid and both front and back circuits at once), this seems like a "safer" solution. At least I shouldn't lose both circuits at once anymore. Comments?

Don, you'll be just fine with the manifold removed. I used a bolt to (1) replace the switch and (2) pin the double-ended piston in the center... that worked for me, but I'm not recommending one approach over the other.

Don,

I can't agree more about getting rid of the distribution valve assembly. IMHO, it's the weak link in the brake circuit. I understand the theory behind it and why the factory had to install it but that doesn't mean the system can't be improved.

After losing the rear circuit on my car last year and the light on the dash not so much as winking at me, I decided to re-design the system much like you describe. In the place of the distribution assembly, I used a T-fitting from the rear circuit to split the front circuit from the MC to the two front wheels.

The new rear circuit just runs straight from the MC to the rear wheels. Attached is a photo of my modified system. If you have any questions, ask away.

Curtis and Simon I can't disagree with you more.
The H valve is a safety device and removing it is a big step backwards.
Not only are your cars less safe they are now also illegal for use on a public road.
Just because it's an old part and failed isn't justification to defeat it's purpose.
You wouldn't disable and remove an old seat belt would you?
You would replace it with a better part I would hope.
There are many options available that would retain the valves intended function and actually improve your braking performance. My opinion is that a combination valve from a newer north american car would be the best choice. Something from a front disc, rear drum vehicle without ABS, would give you all the advantages of RP valves and a metering valve for the front brakes. You also get to retain your warning light and the blocking feature in case of catastrophic failure.
I don't want to come off as a nag but isn't one of our mandates to improve our vehicles? Not to degrade them to an earlier less safe state.
In your case Curtis you need to fix your H valve issue right away.
Your valve has failed and sealing it with a bolt only removes a symptom. The failure is still there.
As your valve continues to degrade both sides of your brake system become connected. This brings about all of the problems with a single brake system only worse.
You will have grossly unbalanced braking as the rear drums start to recieve the same pressure as the front discs.
You also now have opened the door to complete sudden brake failure with only one small leak.
This is what the dual brake system was designed to avoid.
I certainly don't want to offend anyone, just wanting to keep you guys alive.
Lets play safe out there and make our cars better.
Cheers
Fred

By the way Simon I'm in Abbotsford if you ever need any help.



Edited 1 time(s). Last edit at 12/13/2010 02:39AM by DiDueColpi.

Fred, I think your advice is at least half right but I don't think it's right for my situation.

As I said in my post above, the bolt I installed in lieu of the original electric switch pins the double-ended piston in place. Until my seals start leaking, the front and rear circuits remain as isolated from each other as they can be considering they're both getting equal displacement from the master cylinder. I would have re-plumbed my system to look like Simon's, but I got the same effect without spending any cash. Hypothetically, if there were no seals in that manifold at all my brake system would be as "safe" as the single circuit systems MG kept-on installing on their home-market MGBs.

I appreciate the utility of a metering valve for the front brakes... metering valves do two things: (1) they reduce wear on the front brake pads during relatively low-fluid-pressure brake applications, and (2) they prevent the front brakes from locking up on snow or ice. The residual pressure valve I've added on my rear circuit reduces the utility of a metering valve during low pressure stops. I never drive my MG on snow or ice.

Metering valves aren't proportioning valves. At high fluid pressure, metering valves aren't effective and that's the reason you won't find them on race cars. Wheel cylinder sizing and pad material need to be correct to achieve balanced braking. If that's inadequate, an adjustable proportioning valve is the next logical step. Proportioning valves are normally installed to affect (reduce) braking on the rear circuit only. The multi-purpose "combination valves" commonly found on 1970s-1980s American cars have both metering and proportioning features built-in, but I'm weary of replacing the MG manifold with one that includes a proportioning valve for a car that may have a very different weight distribution or differently sized wheel cylinders.

Fred, I think I have pretty thick skin and you're nowhere near offending me. I don't mean to offend you either. My MGB is drifting more and more toward being a track-day car... if I had more cash I'd have dual master cylinders and an easily adjustable bias bar. Although that's what I ultimately want for my own car, I also want to hear other opinions and generally learn about this stuff so I encourage you to point out the flaws in my reasoning.

Hi Curtis,

I don't understand why you would pin the H valve if it was functioning properly.
When it does start to leak you will never know until you have complete brake loss.
When it does start to fail you not only become a single circuit system, you will start to transfer fluid from one master chamber to the other until one runs dry.
Lastly were I am, a brake failure warning is a legal requirement for a vehicle used on a public road.
You and I take good care of our cars and would know when something is wrong.
What about when you sell it? Just add a lawyer and stir over a medium heat.
I know dollars always come into play but any tech inspector that sees a pinned valve will sideline you until it gets fixed properly.
You're right that metering valves are not proportioning valves. They only slow the flow of fluid and have no effect on brake pressure. Used on a rear drum front disc combo they allow the slower reacting rear drums to stay in step with the front discs.
Braking systems should be designed to be balanced correctly from the start. In a perfect world a proportioning valve would not be needed and in many cases it is not used. The stock dual circuit B is a good example. A proportioning valve is not required here and isn't used. But change the engine, the disc size, add rear discs or better pads and suddenly all bets are off and a valve will probably be needed to rebalance the system. Having said that, a proportioning valve is only meant to fine tune a system that is close to correct already. It will never fix a badly matched system. And should never be used on the front brakes for any reason. Many people don't properly understand the operation of the proportioning valve and use and adjust it incorrectly. These valves allow full brake pressure up to their set point. They then start to reduce pressure from that point on only. They are'nt really proportional and the adjustable ones only adjust the fall off point.The rate of fall off is built in.
Residual pressure valves are used to keep air from entering the brake system. A side benefit is they keep your brake pedal nice and high and can reduce pad knock back when used on disc systems.
I have no connection to them but www.stoptech.com has an excellent tech section on brake functions that makes brake selection much more manageable.
Cheers
Fred

Why would I pin the valve? Because when that piston slides from side to side the brakes don't seem to bleed as nicely. (Besides, I understand fluid can possibly leak out through the switch;) The master cylinder has a common reservoir with a partition, so if no fluid is leaving the overall system you can't realistically transfer all the fluid from one side or the other. The fluid would instead flow overtop the partition.

There's no possibility I'll ever sell my MGB, but I suppose someone will get it when I expire. My MG didn't have a functioning brake pressure light when I got it 22 years ago. Most of the wiring was already torn up. You might yet convince me to remove the old manifold this winter... but you probably won't convince me to reinstall a warning switch or to put an idiot light on my dashboard.

Don't worry Curtis I'm relentless.
We'll get that valve switched out yet.
Besides it's the first step towards a proper dual bias bar setup.
Cheers
Fred

This is the first time I've ever heard of that distribution valve/block being described as a proportioning valve. Heard that from someone else today as well. Based on it's construction, I don't see how it actually would "proportion" the two circuits. AFAIK, the theory behind the distribution valve is to move the shuttle when one circuit fails due to an unequal pressure in the block. When the shuttle moves, it contacts the metal tab on the plastic switch, closes the "brake failure warning" light and the driver is made aware of a system malfunction......on paper anyway.

All I can tell you is that when I lost the rear circuit a year ago, the light on the dash did not come on and I know the shuttle inside the valve was good because I installed new o-rings and the shuttle moved in the block. Other than spongier brakes and finally getting a chance to see the rear reservoir empty, that's when I knew I'd lost the rear circuit.

In regards to proportioning, I don't see how my system is any different from a proper functioning brake distribution valve (with working light). The front and rear circuits are kept separate from each other either by the shuttle and its seals or, in my case, the lines running directly from the appropriate reservoirs to the rear and front wheels.

Again, in the original system, when the pedal is pressed, equal pressure is applied to both ends of the shuttle thus keeping it in the centre of the block and not illuminating the light.

This is not a question of money; just my idea on how to fix what I think is the weak link in the system. When I read of owners looking at how to fix a leak in the block by installing a bolt (no finger pointing...;), I wonder why not just replace the o-rings/seals and call it a day. Or do like I did and remove the block altogether. I've had no issues with front or rear wheel lock-up with the car and I'll certainly know when I lose a circuit again.

As for the safety aspect and a future owner, like Curtis, I have no intention of selling the car so I won't have to think about it.

No offense taken either, Fred. That's why we have forums to discuss our cars. I respect your opinion and thoughts but in my mind, my car is no less safe than any other MGB on the road. I know the car, I built the car and I like to think I'll know when something isn't quite right with her.

Cheers
Simon

My 2 cents worth on the subject.

I think we all agree, the front and the rear brake circuits must be kept separate, regardless of what we do with the brake light warning assembly.

If the shuttle seals in the assembly are intact, the brake lines will remain separate, whether or not the switch works. If one seal fails, brake fluid can leak out of the switch; If both seals fail, the front and the rear brake lines will be inter-connected and fluid can leak out of the switch.

If both seals are good, the switch can be removed without any leaking. No need to plug the hole, except as a precaution should a seal fail.

The warning lamp/switch serves no protective function, only a warning that there has been a loss of fluid in one of the brake lines. If the driver is not sensitive enough to sense this, then the lamp is very valuable. If the driver is sensitive enough, then the lamp is redundant.

If you should choose to keep the valve assembly, it would be a good idea to make sure the seals are good, regardless of what you do with the switch. If you should choose not to use the warning lamp/switch function, I think it would be a good idea to remove to remove the assembly and use separate fittings to maintain the separation of the circuits. Without the warning lamp function, the shuttle assembly just adds two extra failure points without adding any compensating benefit.




Edited 1 time(s). Last edit at 12/14/2010 11:48AM by danmas.

I agree w/Dan - my "assembly" has failed twice now, leading to loss of all fluid and all brakes both times. I'm separating the circuits as described by Simon. I do agree that you either try to use the system as designed, or get rid of it completely.

If you live in Canada I think maybe you should keep it. In the US, at least for now, I'm pretty sure you can do as you please. Not sure about the Left Coast, but pretty much everywhere else nobody is going to look, or know what they are seeing if they do. That piece was probably added by legislative fiat anyway, but it probably has served a useful purpose in quite a few cases, alerting owners to a service issue they would have missed, and if properly maintained it will probably continue to do so. Particularly if the car is driven seasonally or occasionally it might be a good thing to have.

But, I removed mine decades ago and never missed it. By the same token, a great number of drivers for decades made do with single circuit systems (and some still do) without a mishap. But it's a wonderful tonic when that mishap does occur. Absolutely nothing in the world like hitting the brakes to stop for a train and going right on across... Instantly puts you in touch with how much you love life. I heartily recommend it, as long as you manage to survive. Yes, a dual system is wonderful, just like a handbrake is, but it only helps a little and sometimes half-slow is worse than half-fast. Better to check that fluid level often and flush the system regularly. Personally I like the fine control of a balance bar and separate systems. A little tweak for dry, a little tweak for rain, but to know you have it right you do have to max out the brakes at least once every time you drive. After all, who's to say the kids didn't climb in the car and turn the big shiny knob?

OK, let me ask, who is for taking all the sports car out of the sports car? We can do that you know, all we have to do is to make every function automatically perfect and maintenance free for life. Cars like that are all the new drivers know. Soon they will drive themselves better than we can drive them, brake better than we can brake, steer better than we can steer, see better than we can see. Is that a sports car though? Where's the fun in that. Maybe it'd be better for every one of us if we just went back to cable brakes, cone clutches, straight cut gears and kerosene headlamps. Try a challenge! There's a sport for a real man.

Not that it wouldn't be fun once or twice, but the fun would fade fast. Give me a key for starting. Better brakes? BAH! Who needs them? Well,... there *are* those dreadful crash statistics. Think I'll go have a look at the fluid level.

JB

Thanks for posting that photo, Dan. I knew I'd seen that one before. I don't know if I'd call the design ingenious but certainly innovative.

As described and working properly, I have no issues with it. As mentioned though, it does have it's share of problems. As we all know, this assemble was created due to tightening safety regulations (along with raised ride height and rubber bumpers). I like the advantage of having two independent brake circuits thus the reason for creating the system I have.

I now know when there's something amiss with my brakes. If the dash light worked properly everytime, I'd leave the assembly in place.

It's been awhile since I had the MGB manifold apart. Maybe one of you can refresh my memory, because I don't think that image (above) is a very accurate illustration of how the MG part was made. I think I recall that the threaded hole for the switch continues all the way down, rather than constricting to a diameter slightly larger than the switch's plunger. More importantly, I recall that the MGB switch sits in the center of a much wider groove so that the piston can move a significant distance before a warning light comes on.

There's definitely no proportioning valve built into the MGB manifold. On manifolds that have that feature built in, there's a bulge on the rear-circuit side with some additional moving parts inside. The question is whether the MG manifold has a metering valve feature built in, and I believe that's possible without any additional moving parts although normally you'd expect to see a pushbutton on the side of the manifold which is supposed to be pressed when you're bleeding the brakes to keep the piston from reaching the full extent of it's travel on whichever side has low pressure. (Couldn't the tip of a bolt temporarily installed in lieu of the warning switch serve the same purpose?)