MGB MGB brake problems

[R6MGS]

Hi Everyone, over the last few weeks I've been working on taking one of the 78 MGB's out of storage. I originally had some problems with the brakes binding up after being driven for about 10 minutes or so. Since then I have done the following;
-Rebuilt rear wheel cylinders
-rebuilt master cylinder
-new front flex lines
-new rear brake hose(chassis to axle)

After all of that I am still experiencing binding once hot. The pedal gets firmer and firmer to the point where it becomes locked and nearly impossible to push down at all.once this occurs the car cannot be moved until the brakes cool down.

I am pretty much out of ideas, other than possibly the front calipers overheating and becoming locked? or maybe something to do with the brake booster as I am not very familiar with the internal workings of such a device.

Any suggestions or advice you can offer would be greatly appreciated.

Thanks

 

[DrEntropy]

After this condition is present, jack up the front-end and have a helper push the pedal and see if the front wheels still spin freely or have any caliper action... that's a weird condition, BTW.

If the booster weren't getting vacuum, the pedal effort would be greater but still stop the car. If I'm reading it right the pedal becomes rock hard and NO amount of pedal effort makes any difference?

 

[R6MGS]

Sorry for the lack of clarity...Once the pedal becomes almost 'locked' the brakes are also all locked on. The car cannot be moved, even using engine power, until allowed to sit and cool down. Once cool the brakes perform normally until reheated.
It almost feels like the system is pressurized and cannot release.

 

[Scott_Hower]

*cough* PDWA *cough*

 

[rick_ingram]

This is assuming that the brake m/c is working properly.

Scenario One: If the brake m/c has been over-filled, the brake calipers can refuse to release as the fluid heats and increases in volume....no where for it to go....pads remain in contact with rotors.

Scenario Two: Just because the flex hoses are new does not mean they are good. They may have already swelled/collapsed.

Scenario Three: The vent hole in the cap to the brake m/c has become plugged.

Scenario Four: The calipers themselves are sticking.

 

[Scott_Hower]

Scenario Five: the shuttle seals in the PDWA are shot.

 

[DrEntropy]

I vote for #5, with #2 as a second choice.

You back home, Scott?!?!

 

[Scott_Hower]

I vote for #5, with #2 as a second choice.

You back home, Scott?!?!

Yes. Kissed the ground at JFK when I arrived late July.

Which wheels are locked... Opposite corners?

 

[Guest]

I had a brand new Pontiac Bonneville in 1993 that came into the dealer doing this exact thing.... 6 month old car had only 700 miles on it because the techs could not figure out the problem.... Finally, I took the job, because GM was faced with a buy-back. I took it out on the road with a 15mm wrench... sure enough, after only a few miles, the car came to a stop without me touching the brake pedal.... I got out and backed off the 2 bolts holding the master onto the power unit abot a turn each, VOILA! The brakes released. I took it in to my work stall, removed the pushrod, ground off about .040 inches (1 MM)... problem solved... Previous tech had replaced calipers, master cylinder, power boost unit, flex hoses... all to no avail. It's a matter of step-by-step process of elimination. I would suspect front calipers first.

 

[R6MGS]

What is a PDWA?? I am not sure which wheels are locked....Definately both front ones, not 100% sure if the rears are locking as well, but it feels like they are.

BTW; this is very helpful, thanks guys

 

[DrEntropy]

For the hydraulic system to build pressure then HOLD it, something odd is occurring. Are you SURE all the seals in the master cylinder are in their correct orientation?

The PDWA is a diabolic device in the braking circuit mainly to warn you of hydraulic failure on one or the other of the two braking circuits. It has a switch for the "brake warning light" mounted in it and is engineered to cause you more grief than aught else. It can be "shuttled" to center (or checked to ascertain it is already centered) by removing the electrical switch and with a screwdriver centering the "puck". If fluid escapes from the switch mounting hole it needs new seals... I'm not so sure it could cause the condition you describe if it goes to one side or the other tho.

The brake hydraulics are in two seperate MC systems, staggered to allow you marginal braking on one front and one rear corner, opposite sides. The PDWA should be mounted to the inner wing of the engine bay on the driver's side. Four lines and an electrical connector go to it.

Sherman makes a point: process of elimination, start with the calipers. If you cannot EASILY squeeze the pistons back into their bores with a c-clamp (and an old brakepad) try opening the bleed screw and see if they go in more easily... this eliminates the calipers if the pistons go into their bores smoothly. And opening a bleed screw with the condition present may produce a sudden release of fluid under pressure as well.

As a caution: you may need to hold each in its respective bore while exerting force on its opposite.

I keep goin' back to the MC seal idea. That or those flex-lines being internally swollen.

 

[R6MGS]

Welp, found the problem....Back a winter or two ago we had to do some work on the clutch slave cylinder, in order to access it from the footwell the brake pressure switch was removed. Then later reinstalled and threaded in too far. This was keeping the pedal from coming back up all the way, keeping pressure in the system.

Thanks for all the assistance with this....now that this is sorted it's been resurrected from 2 years storage and everything else seems great.

 

[rick_ingram]

Glad you found the culprit.

Kudos...I was wondering...

 

[PAUL161]

Now that's one to put in the memory banks! Who would have thought that screwing that switch in to far would do that.

 

[DrEntropy]

eeeep. Never gave a thought to misaligned/maladjusted brakelight switch. One check of pedal freeplay should have been "telling".