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MG Midget and Sprite Technical - Brakes pumping up

Hi all, My brakes have been giving me problems for some time now so I want to ensure I am making the correct assuptions before tearing them all apart again.

The front brakes start to bind after some time and usage, when parking the car up, the front wheels are hot and I don't need the handbrake, this goes away after a few hours.
I rebuilt the master a few years ago and I wasn't convinced the piston seals were of the best quality as they seemed very tight in the bores and I'm starting to think this is the problem. I stripped the master again last year, cleaned it all out especially the little hole that I've seen mentioned in other threads. I then thought I had my pedal adjustment wrong as the pedal had zero travel after some time, to experiment I backed this right off to the point where the brake pedal travel is huge from cold and then gradually decreases until it is almost solid and the brakes bind again. If it were related to flexible hoses and calipers I would not have the pedal travel moving correct? so it must be a problem with the master and the adjustment of the pedal travel/pushrod right?
Anything else I can check? how does the residual pressure return to the master reservoir when you release the pedal?

Oops, forgot to mention its a Frogeye with orig master running discs.

Thanks

Mark
Mark M

Hi Mark,
I can't answer your question but I noticed that your email address has been bouncing back (If you are the mMark that I think you are). Has it changed?
Gary Lazarus

Nothing "pushes the residual pressure" back to the master cylinder other than the disc scuffing the pads as it rotates. Other than that, the pads remain in light contact with the discs at all times.

The usual cause of front callipers progressively binding on is perished flexible hoses acting as a one-way valve for the fluid. But I suspect that you have already replaced the hoses. If not, then I would certainly do so as your symptoms match that particular problem.

It is also essential to get the rear brakes correctly adjusted before attempting to resolve any front brake problems. Excessive rear shoe travel will take up all of the fluid movement and play havoc with the operation of the front callipers.

If you have dealt with these issues, then I would suspect a faulty mastercylinder, maybe wrong parts or wrongly assembled.

Guy
Guy Weller

Hi Gary, Yes, its me ;o)
Try mark dot maguire1 at sky dot com

Guy, The hoses have not been replaced for at least 10 years but then the car lives in a garage and does very low mileage, I can replace these but would this contribute to the pedal 'pumping up'?
The rear shoes should be fine but I'll check these over anyway, thanks.
I'll search out a diagram of the Master assembly (unless anyone has a link to hand?) and order a new seal kit anyway just to be on the safe side. Any recommendations on make to ensure its decent quality?

Mark
Mark M

ISTR when converting to discs and using the same m/cyl there is a small part that needs to be left out, have you checked the archives as this has been discussed here before....
David (davidDOTsmithAT stonesDOTcom)

Mark
Hoses: Yes it would. 10 year old hoses could well be the problem. The rubber of the hose deteriorates and swells internally, restricting the bore of the pipe so that it acts like a one-way valve. When you press the pedal fluid is forced past the restriction (foot pressure will easily produce 400 lbs psi) so the brake pads grip the disc. But then as there is nothing to force the fluid back again past the restriction, when you release the pedal the pads remain pressing against the disc. Next time you press the pedal the same thing happens again, and again etc with the brakes binding on harder each time you brake and probably overheating. If parked and left the fluid will gradually seep past the restriction, releasing the pressure. 10 year old pipes are definitely old enough for this to happen - even if the car hasn't been driven much in that time.

A long pedal stroke when you first press the brakes is most likely due to slack rear brakes as I explained earlier. Or it could be to do with brake push rod adjustment or even worn front wheel bearings. But my money is on poor rear brake adjustment.

Guy
Guy Weller

Could simply be a case of the master cylinder actuating rod being poorly adjusted.
Wouldn't argue with Guy but this type of problem is caused more by the master cylinder not returning far enough to clear the tiny hole to the fluid reservoir than a calapsed hose, although that does occur. IMO :-)
Robert (Bob) Midget Turbo

Mark,

David Smith above mentioned the valve that should be left out of the drum brake master cylinder when used with the disc brakes. I had the same setup on my frogeye, it came with drums and I converted to discs and left the drum MC as it was. I didn't know about the valve removal and never had an issue. The only locking brake issue I had was with the correct disc brake MC fitted and that seemed to be due to a combination of very worn pads and discs, I replaced both and the problem went away.

The only collapsed flexible hose issue I've encountered was on a mini clutch slave. You could disengage the clutch easily but it took about 10 minutes to re-engage. Not encountered that before or since, so it was puzzling at the time but easily sorted once diagnosed.
David Billington

I've run across internally swelled lines a couple of times, once on the rear and once on a left front as well as a similar problem with a rubber fuel line years ago. Now I just install Goodridge type lines and never worry about them again. Well worth the investment. I would't worry about the residual pressure check valve too much, it should only leave about 2 to 4psi pressure in the lines which the front rotors should easily be able to overcome without causing the pads to drag. You should keep the valve in place for the rear brakes though, keeps the rear shoes from retracting too much and causing a long pedal. One option would be to remove the valve from the master cylinder and then install a residual pressure valve in the line to the rear axle only. That's the ideal way to handle it with a disc brake conversion and still using the original master cylinder.
Bill Young

The brake pedal adjustment/pushrod adjustment is causing the long travel, I mentioned in the first post that I backed this right off to see if this made a difference, all it seems to do is prolong the time before the slack is gone and the brakes are binding.

The valve is already removed, I remember some interesting debate about this many years ago.
So last question I think is if the front hoses could be a problem, I accept that this can cause the bind, but how does this cause the pressure build up in the remainder of the system and therefore the lack of pedal travel?

Mark
Mark M

oh, and what does the little hole in the M/C do from the reservior into the bore?

Ta

Mark
Mark M

The little hole Mark?

Well when the master cylinder is completely released back the cylinder has a small hole in the top that connects the cylinder to the fluid reservoir. Thus pressure releases into the reservoir.As the pedal is operated the piston in the cylinder pushes fluid past this hole and into the system to pressurize the brake circuit. If the piston does not come back far enough to clear this hole and release prssure then heat created by the action of braking causes epansion of fluid and molecules of air in the system that pressurises he braking circuit making the brakes drag.

I wish I had the ability to eplain things clearly. :-((
Robert (Bob) Midget Turbo

The small hole is the fluid return port which allows fluid to return to the resevoir when the pedal is released. The piston in the cylinder has to return far enough to open this port or the brakes will hang on as the fluid can't return out of the cylinders and calipers. An improperly adjusted push rod (too long) or on later models an improperly adjusted brake light switch can cause this problem.
With hoses that are internally swollen they act as one way valves. The fluid from the master cylinder can flow through the restricted area because of the pressure it's under from your foot. When the pedal is released the only pressure to force the fluid back into the resevoir is that small amount from the brake return springs on the rear brakes and the action of the front disc taping the pads back into the caliper. Not nearly as much pressure so the fluid won't transfer. The pedal does however return to it's normal position and the master cylinder bore refills from the resevoir. The next time you apply the brakes the pedal is "hard" because there's no place for the fluid to go, the calipers and wheel cylinders are already full. Until the fluid can seep slowly back through the line restriction the pedal will be short and hard. Once the pressure in the calipers and cylinders has been reduced everything works as normal for one more cycle.
Bill Young

I did my '63 (3/4" bore, front disc) and ended up having to scrap it for a 7/8" since the 3/4"'s bore was too pitted. During the several rebuilds of both I encountered quite stiff resistance when the springs try to push the pistons back, new seals quite tight. Now on a pressurized working system I imagine this isn't an issue as the pressure helps the springs and all. I'd replace the hoses, adjust the rear and reset the master where it should be as far as pushrod clearance and all that. I'd bet the "failure to return" syndrome goes away.
J Van Dyke

I've taken this opportunity to replace the front hoses with braided versions and ordered a new master rebuilt kit, hopefully this will solve the problem.
Thanks for all your input.

Mark
Mark M

Mark A suggestion. Do one thing at a time then you will find out where the problem was.

Guy
Guy Weller

I agree with Guy, make one change at a time to be sure where the problem was and to possibly save yourself some time and money as well. From your description it does sound like the master cylinder is working fine so I'd go for the lines first.
Bill Young

This thread was discussed between 18/08/2009 and 20/08/2009

MG Midget and Sprite Technical index

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