Back from vacation so time to once again try fixing my locking front brakes..nothing has worked so far

[limey222]

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.

[88v8]

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]

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.

[Moderator]

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.

[BlownMGB-V8]

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]

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.
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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]

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

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[Moderator]

Probably not what you want to hear but here's a simpler, cheaper, lighter weight way to get  reduced pedal effort:
<a href="http://www.britishv8.org/Articles/Budget-Brake-Mod.htm">increased leverage</a>

[limey222]

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]

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]

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]

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]

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]

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.  



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[DiDueColpi]

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]

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]

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]

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]

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]

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

[limey222]

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.

[limey222]

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

[RMO 699F]

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

[limey222]

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

[BlownMGB-V8]

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