Steering, Suspension, & Brakes

tips, technology, tools and techniques related to non-driveline mechanical components

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limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 22, 2015 11:12PM

Just a refresher, I can drive my car around the block and by the time I coax it back to my garage the front discs are locked up again. Have to crack open front bleeder valve(s) to release pressure and then use only the hand brake to get home. BTW the rear drums are fine.

Your suggestions that I have tried so far:

New master cylinder
Spacers installed on master cylinder retaining bolts to ensure 1/4" pedal free play before piston compression
Multiple bleeds
Replaced PDWA with reconditioned unit from UK with new plunger switch. (Checked and shuttle spool stays in mid-position).
Tried replacing plunger switch with modified 3/8"-24 bolt machined down at one end to replicate plunger thereby locking shuttle in mid position...same results
Backed off and even removed the brake light switch

I'm thinking of bypassing the remote servo unit which apparently only serves the front discs to see if that is causing the problem.
The only other thing I can think of is that the front calipers are seizing and not backing off but that doesn't explain how things are rectified when the pressure is released by cracking the front bleeders.

This is really disconcerting, I've spent a year doing my conversion and rebuild and it promises to be really something, but so far I have only got far enough down the road in 2nd gear before the brakes lock. Please can someone help me out here.



Edited 1 time(s). Last edit at 09/22/2015 11:39PM by limey222.


88v8
Ivor Duarte
Gloucestershire UK
(902 posts)

Registered:
02/11/2010 04:29AM

Main British Car:
1974 Land Rover Lightweight V8

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: 88v8
Date: September 23, 2015 04:44AM

Errrm.
If there is air in there it would heat up and expand and could makes the brakes drag but I'd be surprised if it happened that quickly, especially as presumably you wouldn't be using the brakes that much or that hard on a short drive. And anyway, you've bled it.

So the next suspect is the hoses but they are new.

Now back to the m/c. Did the new one come from the same source as the previous?
Just thinking that if the master isn't retracting enough to uncover the front return port, that will soon lock the brakes as the fluid will only go one way. You have free pedal play and no inline servo, so that shouldn't happen, but if the seal is too long - faulty manufacture - it could. Would need to dismantle the master to check.

The servo - the remote servo unit has a slave cylinder attached that actually works the brakes (helped by the servo). The cylinder operated by the brake pedal acts as the master to this. So, yes, you need to look at the slave in the servo as well, could be the same problem of inadequate return. Plumb it out of the system or strip it to check.

Final thought - what brake fluid are you using?

Ivor


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 23, 2015 08:56AM

I'm using DOT 4 brake fluid.

The meter cylinder was labelled as a new TRW Lucas MGB 68-74 Made in UK PMA700 coating $136 from sportscarpartsltd on Ebay. I assume that it isn't one of the cheap knock-offs.



Edited 1 time(s). Last edit at 09/23/2015 09:24AM by limey222.


Moderator
Curtis Jacobson
Portland Oregon
(4495 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: Moderator
Date: September 23, 2015 10:06AM

Are we talking about a US market car? LHD and dual circuit brakes from the factory? It would never have occurred to me to fit a remote servo, and I can't help but be skeptical of them. If someone held a gun to my head and told me I must install power brakes on my MGB, I'd use the late model (non remote) system. Can you post pictures of the servo's installation? I've only noticed them on MGC. I'm curious to see what yours looks like.



Edited 1 time(s). Last edit at 09/23/2015 10:11AM by Moderator.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6070 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: BlownMGB-V8
Date: September 23, 2015 10:25AM

I'd guess the remote servo is your problem. Now I've not torn one down so I can only guess at how they work, but at the very least it has to have a piston that operates the vacuum valve to apply boost to the servo piston. There may be an adjustment on that primary piston. Worth looking at.

Jim


DiDueColpi
Fred Key
West coast - Canada
(1309 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: DiDueColpi
Date: September 23, 2015 01:11PM

Initially I thought Michael was using the conventional braking setup.
But for everyone out there that is trying to figure out just what is going on.
Here are some pictures of Michael's system.
These parts are from an XKE so not visually the exact same but they operate in the same way.
IMGP9624.JPG
IMGP9625.JPG
IMGP9626.JPG
The bigger unit is the slave and the smaller one is the master (connected to the pedal)
The master builds pressure that is transferred hydraulically by a tube to the slave to push the piston in the slave which in turn supplys pressure to the brakes. The small valve at the end of the master houses a vacuum regulator that supplies vacuum to the slave pot (the big can on the slave) when the brake pedal is depressed. The canister on the slave then pushes on the piston in the slave along with the hydraulic pressure from the master to give the brakes power assist.
The smaller systems have all the vacuum valves built into the slave but the operation is the same.
As you can see it's a very convoluted setup with multiple opportunities for failure.
The most failure prone area is the vacuum regulator valve. A small leak will turn your brakes on at any time.
Second is the slave piston not fully returning.
Either one will cause the problem.

Cheers
Fred


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 23, 2015 02:19PM

Here are photo of the actual servo in my car. You can see that I have just removed it from the brake system as a trouble-shooting measure. I now have to bleed the system to see if it's removal has solved anything. In the second photo you can see the new "bypass" line I have just installed where the servo used to be connected

IMG_0837.JPG
IMG_0836.JPG



Edited 2 time(s). Last edit at 09/23/2015 02:22PM by limey222.



Moderator
Curtis Jacobson
Portland Oregon
(4495 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: Moderator
Date: September 23, 2015 03:51PM

Probably not what you want to hear but here's a simpler, cheaper, lighter weight way to get reduced pedal effort:
increased leverage


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 23, 2015 04:18PM

Curtis,

I have no problem doing the suggested mod down the road, it's a good idea. However I just want to get the brakes to stop locking on first.


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 23, 2015 04:29PM

OK, got the system bled, the pedal is now nice and firm. Next I checked the position of the "shuttle" in the PDWA and it was in the center so I thought I would just try and spin a front wheel by gripping the studs (the wheels are still off), bad idea they wouldn't move. I put lever between two studs and could then I could slowly rotate the wheel. After that and with a lot of force I could slowly rotate the wheel by hand without the lever. Something is still wrong here, I didn't even attempt\t to put the wheels back on and test drive as I've been that route before and the front wheels always bind up after a block or so, guess I am back to square one.
Could the new master cylinder be faulty or could the disc cup seals have melted and are somehow preventing the calipers from returning to normal position. Bear in mind that by releasing the pressure by cracking the bleed screw the callers release and the wheels rotate again until the brakes are used again. Surely if the seals were shot that wouldn't happen.


mgb260
Jim Nichols
Sequim,WA
(2120 posts)

Registered:
02/29/2008 08:29PM

Main British Car:
1973 MGB roadster 260 Ford V8

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: mgb260
Date: September 23, 2015 06:34PM

Michael, On your thread on the other board a guy had the same problem. It was the booster. Or you could have debris in the master?


DiDueColpi
Fred Key
West coast - Canada
(1309 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: DiDueColpi
Date: September 23, 2015 07:01PM

Michael,
Looking at your picture.
I think that you have the brake lines reversed at the master cyl.
The residual pressure valve that should be maintaining line pressure for the rear drums is holding the front calipers on instead.
Reverse them and I bet you'll be fine.


Charles
Charles Long
McDonald, TN
(177 posts)

Registered:
09/15/2013 08:54AM

Main British Car:
1966 MGB V6 1994 Camaro 3.4L 60V6

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: Charles
Date: September 23, 2015 09:43PM

Fred may be correct about the line routing. The residual valve should be on the rear brakes.
If I get to point a finger it would be at the new Moss master cylinder. I installed one on a late B and it was defective out of the box. Moss did replace it , but not the labor and frustration.


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 23, 2015 10:31PM

Fred,
Thanks for the comments but they have left me a little confused. Please review the attached diagram and my photo, they appear to match each other or am I reading it wrong.
I took the car again tonight just around the local residential streets, Sadly within a few hundred yards the front brakes began to bind. Just trying to coax it back home I noticed that the binding was progressively getting worse until I had to really gun it up the drive and into my garage. I tried something different this time and cracked the rear connection on the master cylinder, again a little spurt of fluid and the brakes released.



MGB brakes 2 (1).jpg
IMG_0837.JPG



Edited 1 time(s). Last edit at 09/23/2015 11:38PM by limey222.


DiDueColpi
Fred Key
West coast - Canada
(1309 posts)

Registered:
05/14/2010 03:06AM

Main British Car:
I really thought that I'd be an action figure by now!

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: DiDueColpi
Date: September 24, 2015 01:50AM

I agree Charles. If the lines turn out to be correct then it almost has to be the master cyl.
It is very odd that releasing pressure on the rear would release the front?



limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 24, 2015 09:02AM

Hi Fred,

In regard to your comment

"I agree Charles. If the lines turn out to be correct then it almost has to be the master cyl.
It is very odd that releasing pressure on the rear would release the front?"

Are you referring to my last post where I said that for the first time I tried cracking the MS rear fitting instead of the bleed fitting at a front wheel?

After reviewing my last post are you in agreement that the way mine is connected matches the diagram or do you still feel that it is incorrectly connected?

At this point I willing to try anything. If I have to buy another new master cylinder because mine is faulty then so be it. The car feels so good to drive (even in just the first three gears) that I am really desperate to get this annoying problem finally solved.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6070 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: BlownMGB-V8
Date: September 24, 2015 10:30AM

Could be the shuttle valve shifted when the rear line was cracked?

I had a rear brake master on a motorcycle once that did this. The seals had been changed and it had a longer cup that did not uncover the bleed port at full return. I fixed it by drilling a new bleed port and never had another problem.

Are the lines for front and rear different sizes? If so it would be hard to mix them up, though I suppose anything is possible.

If the pressure does not release the fault has to be in the master cylinder, provided its piston is not being held in a partially depressed position. (or is sticking before retracting fully) The reason is that there is a very small drilled hole that the seal uncovers when fully retracted which connects to the reservoir. This allows the circuit to refill if there has been any loss and releases any residual pressure. The only way for pressure to build up is if this port is blocked off, although a residual pressure port in the line as is sometimes used (sometimes as a part of the port fitting) can create residual pressure. This is usually a lightly loaded spring valve of some sort. It is not a necessary part of the braking circuit, correctly sized and adjusted brakes work just fine without them.

Jim


limey222
Michael Cubbon
Portland, OR
(129 posts)

Registered:
08/01/2015 12:55PM

Main British Car:
1969 MGB GM 3.4L V6

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: limey222
Date: September 24, 2015 11:09AM

The other condition that I noticed last night after I got the car home was that the shuttle valve in the PDWA had moved from the center position. The electrical switch had closed because the plunger had been operated. There are no leaks anywhere in the system so what could cause that to happen? It had been in the center position after bleeding and before driving which would have indicated a balanced pressure on the two circuits.

I've had people tell me to remove the switch completely and replace it with a 3.8" - 24 bolts that has been ground down to a point so that it can lock the shuttle valve in the center position. Does that wok and surely it is just ignoring the real cause of the problem.


Nexxussian
Erik Johnson
Alaska
(62 posts)

Registered:
04/20/2015 10:32PM

Main British Car:
1974, MGB, Citroen Color Rover V8

Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: Nexxussian
Date: September 24, 2015 01:24PM

Michael, if you try driving it again, and experience the brakes locking up, try removing the clevis pin at the brake pedal.

If the brakes release, you've found your culprit (too long pushrod / to shallow pushrod cup in aft MC piston).


For whatever it's worth, I have driven a car with the wrong residual pressure valve in the front brakes ( hot rod with MC under the floor, was supposed to have 2 psi "disc brake" valves, got 10 psi "drum brake" valves by mistake).

It wasn't anywhere near as much braking force as you describe (locked wheels) it felt more like the rolling resistance of bias ply tires VS radials.



If I read your earlier post correctly*, that eliminates a clog in the plumbing.



* I read that you had released the brake pressure by cracking loose the fitting for the front brake circuit at the Master Cylinder.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6070 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far
Posted by: BlownMGB-V8
Date: September 24, 2015 02:13PM

Michael, you have to go about this systematically rather than bouncing back and forth between a half dozen parts and questioning everything.

For instance, you say the shuttle valve moved. But you also said you cracked the rear bleed to release the front brakes. Wouldn't that be exactly the conditions under which the shuttle valve is expected to move? So perhaps it is operating as it should. The proper thing to do is to check and see if it moved in the direction of the rear brake circuit. If it did, you know exactly why and it is working properly.

It seems obvious that the problem is a buildup in pressure in the front brake hydraulic circuit. The only 2 ways for that to happen is for the volume of fluid in the circuit to increase, or for the system capacity to decrease. Is it fair to assume that there is not some mechanical thing compressing your caliper pistons and simultaneously clamping the pads to the rotors? Then that leaves an increase in fluid volume.

Which means fluid is traveling from the reservoir or the rear brake circuit (the only sources of extra fluid) into the front brake circuit and not returning. That indicates the effective existence of a one way valve, or check valve and an otherwise closed system.

Since the function of the master cylinder is to open a port when the cylinder is retracted (brakes released, the internal spring pushes the piston out to the cylinder end stop under the boot) between the reservoir and the brake circuit, when the brakes are released, BY DEFINITION OF THE MASTER CYLINDER DESIGN, the system is open to the reservoir!

Which means, THE ONLY WAY for the circuit to be pressurized is if the master cylinder has failed! (Go on, tell me I'm wrong here.)

Jim
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