Steering, Suspension, & Brakes

Jim,
Thanks once again for your clear and precise posts. Please understand that I have been fighting the problem for almost month now. I am as mechanical engineer by trade and am used to approaching problems in a systematic manner. From my last few posts it may seem that I'm bouncing all over the place but in fact I have previously made a very structured approach checking and eliminating potential causes. I am still very jet lagged from my return trip from Europe so maybe my thinking is still a bit clouded. I didn't crack the rear connection to the master cylinder until AFTER I had discovered that the shuttle valve had moved and I did it only so I could push the car back into my garage.
I finally got an answer back from the company I purchased the master cylinder from. He assured me that they had sold ver 2,000 of them with no returns. He owns 4 MGB's himself and is very familiar with them. He doubts that the cylinder is faulty himself and offered other possible causes for the problem, all of which i had already covered. He seems willing to replace the cylinder if I push him hard enough. He still feels that the problem is in the calipers themselves although I'm not convince since they move easy enough when pressure is released.

I now have a replacement master cylinder on its way to me, should get it in time for the weekend.

Michael....after an incident where the front discs lock up...do the front wheels "free up" after the car sits for several hours?

Yes, they do tend to free-up after several hours

Apologies Michael, no intention to offend. Perhaps I was more gruff than was called for.

The thing about the bleed port in the M/C is that the location tolerance is pretty critical. As an engineer you will appreciate that a stack-up of tolerances can and sometimes does result in an assembly that will not function correctly. My opinion is that it was just your bad luck to get the 1 in 2000 that failed and wasn't caught.

Jim

No offence taken Jim, and just my luck to get a bad one. This project has been a comedy of errors in many ways, at least from my wife's viewpoint. She can't understand why I undertook it in the first place and just sees me being frustrated and disappointed all of the time, but hey, its the nature of these projects. She doesn't know that the old engine, a Ford 2.8 V6 and 4 speed box was on its last legs. I can tell just from the limited driving I've done with the GM 3.4 and T5 combo that its going to be amazing in comparison to the old setup. I just want so badly to get it up and running while there is still some warm, fall weather driving to be had.
Fingers crossed that the new master cylinder will do the trick. I have a set of recondioned caliper assemblies on hold with a local supplier just in case it proves to be them but I doubt very much that it is. I've spent so much money on this car so far, that it would be silly to short change it so close to it being finished if i proves necessary. The only other outstanding work will be to hook up the tach with a signal from the. coil pack.

Michael, three times in my lifetime I have experienced front disc brakes locking up after a short drive in the MGB...each time I replaced both front flexible brake lines and the flexible line that goes to the rear....it cured the problem each time. The situation was the same in that after a few hours after the lock up event, the fatigued/collapsed hoses would relax and let the brake fluid back into the system and the wheels would spin freely until I drove again and applied pressure to the brake pedal. Many on this forum have much more knowledge about this issue, just relaying what has worked for me in the past.



Edited 1 time(s). Last edit at 09/24/2015 10:21PM by RMO 699F.

Mike,

I appreciate your comments but my stainless braided hoses are only 6 months old, I can't believe that they are causing the problem. My problems only started after installing a new master cylinder.

OK, getting vaguely suicidal here, installed the new brake master cylinder, two of us bled these system. The other guy has worked on MG's for years.
Took off around the block (short blocks) by the time I got back to my garage I could feel the front brakes slightly dragging. Around second time and they got progressively worse....same old problems, had to really gun it to get back up my drive. When we were bleeding the system the brake cups were moving OK, he could see them and I could here them from inside the car. The shuttle valve has also switched off mid-position again indicating a pressure differential.
The lines are not hot so fluid expansion is not the culprit. The problem is progressive, the more I use the brakes the worse it get until full lock up. Cracking the front bleeders releases the brakes, what the hell can be causing the pressure build-up and not allowing a return to "tank".
No crushed lines, no collapsing hoses, sure two cylinders in row from different suppliers and manufacturers can't be bad.

There is the slight possibility that the trap valve in the MS is installed backwards (spring on outside of the valve. People are telling me on different thread that it should cause this brake lock problem but I'm going to take things apart and make sure that its correctly installed.



Edited 1 time(s). Last edit at 09/26/2015 04:48PM by limey222.

Is it possible the shuttle valve is somehow acting as a check valve in the front circuit? The only thing that that seems to make sense to me is that there may be some configuration where the port to the M/C gets blocked. I bring this up only because of the way the M/C is routed into one of the side ports. The only two possibilities there that I can see might be if the piston was installed backwards or if the piston's seal location is not in spec so that it blocks off the M/C port.

Jim

Jim
it turns out that it was that darn small trap valve and spring that was installed backwards (spring on the outside) that was causing the problem. So both of the new master cylinders are good. I removed the darn thing completely as some people have been running without one for years with no detriment.
The brakes are working perfectly now.



Edited 1 time(s). Last edit at 09/28/2015 01:00AM by limey222.

Glad to hear that. What an ordeal, huh?

Jim

Yes indeed, many hours wasted and dollars spent all because of maybe a 25 cent part. Hopefully others will learn from my experience in the future.