More Sticky Brakes

[codekiddie]

For the second time since ownership of my 1972 P6 3500S, the brakes locked on :-(

The first time I had only owned the car for a few weeks, and the recovery service that came with the 1 year warranty covered a roadside repair if possible.

They found that the front nearside pads were stuck fast, and simply released them, cleaned everything up, applied copper grease etc, and all worked fine again. When I returned home I did the same to the other side, and for the next six months I had no problems, in fact I was out in the car the day before the second occurance.

The second time it happened I was very close to a garage, and I eventually managed to coax her onto their brake tester which clearly showed both front brakes were locked on. The same cleaning procedure was carried out, and again all was fine, and I put it down to possibly lack of use through the winter, the garage suggested new disks, pads and calipers :shock:

Having now read through all the relevant topics covering such locking brake behaviour, I have come to the conclusion that the problem may lie with the air valve piston of the servo not returning after heavy braking.

Again, having read so much useful information on here, I see that there may be a modification to the spring in use on this valve. As yet all I have removed is what the Repair Operation Manual calls the air filter of the servo unit, and found that the small spring shown in the manual is not present.

My first question is, if the modification spring has been added, would this small spring be removed, or have I lost it?

I understand, from a reply by quattro in another thread, http://www.classicroverforum.com/viewtopic.php?f=14&t=10894&start=17, how the servo system works, and if the air valve piston sticks open then you get assisted braking even without the brakes being applied.

My next question would be, if I have understood the operation correctly, the non-return valve of the servo is what keeps the vacuum after you switch off the engine, if I remove this valve having switched off, should my brakes then release?

It would be a quick test I could do if it occurs again.

My last question would be, since simply releasing the brakes and cleaning the pads etc... seemed to fix them on both occasions, might I be wrong with the servo diagnostic. It just seems odd that both front brakes were locked at the same time, the second time it happened at least, or could the action of pushing back the caliper pistons have somehow forced the air valve piston back into place?

On neither occasion was the servo touched, or the rear brakes tested to see if they too were locked.

Thanks in advance.

 

[harveyp6]

If it's a servo fault it will affect all four brakes, not just the fronts. It could be the front flexible hoses not allowing the fluid to return and so holding the brakes on, or just that as the edges of the pads have been cleaned up several times they now corrode and stick quicker than ever. The way to prove which of the two it is would be to slacken the bleed nipples the next time it happens, if that frees them off it's the hoses, if it doesn't, its the pads or caliper pistons sticking. Before doing either, see if the rears are affected as well.

 

[SydneyRoverP6B]

Hi Phil,

The little spring within the air control valve that sits under the filter cap, is not removed with the addition of the modification spring. If yours is missing, then you need to obtain another air control valve. The modification spring was added so as to prevent the brakes from sticking on following light brake applications.

If the air control valve is unable to return following a brake application, then the air at atmospheric pressure which has been admitted to the rear of the vacuum chamber, will not be evacuated, hence the brakes remain locked on. If you pull out the check valve from the vacuum chamber, this will equalise the air pressure within the two chambers, and the brakes will release.

The air control piston must be able to return following a brake application, otherwise the brakes will also stick or indeed lock on. At the base of the cylinder which retains the piston there is a tiny brake fluid bleed hole, the diameter is much like a pin hole. If this become blocked by heavy use of rubber grease or other foreign material, then again the control piston will be prevented from returning and the brakes will lock on.

Ron.

 

[codekiddie]

If it's a servo fault it will affect all four brakes, not just the fronts. It could be the front flexible hoses not allowing the fluid to return and so holding the brakes on, or just that as the edges of the pads have been cleaned up several times they now corrode and stick quicker than ever. The way to prove which of the two it is would be to slacken the bleed nipples the next time it happens, if that frees them off it's the hoses, if it doesn't, its the pads or caliper pistons sticking. Before doing either, see if the rears are affected as well.

Thanks Harvey. I am hoping to get out later in the week to try and make it happen again, so I can check to see if the rears are indeed affected as well. Thanks for the other pointers if it is not a servo fault.

The little spring within the air control valve that sits under the filter cap, is not removed with the addition of the modification spring. If yours is missing, then you need to obtain another air control valve. The modification spring was added so as to prevent the brakes from sticking on following light brake applications.

Does anyone know if the Air Control Valve kit from JRW includes this spring?

If the air control valve is unable to return following a brake application, then the air at atmospheric pressure which has been admitted to the rear of the vacuum chamber, will not be evacuated, hence the brakes remain locked on. If you pull out the check valve from the vacuum chamber, this will equalise the air pressure within the two chambers, and the brakes will release.

That was what I was hoping for. A fairly simple test, and an easy way to limp home, (albeit without assisted braking), if that is the fault.

The air control piston must be able to return following a brake application, otherwise the brakes will also stick or indeed lock on. At the base of the cylinder which retains the piston there is a tiny brake fluid bleed hole, the diameter is much like a pin hole. If this become blocked by heavy use of rubber grease or other foreign material, then again the control piston will be prevented from returning and the brakes will lock on.

Thanks for the additional pointer Ron.

 

[SydneyRoverP6B]

codekiddie wrote,...

Does anyone know if the Air Control Valve kit from JRW includes this spring?

They are a Lockheed item Phil, the spring is part of the business just like all the other bits. Having said that, JRW may offer a different brand, so can't answer your specific question. At a guess though, I would say yes, as it won't work properly without it.

Ron.

 

[codekiddie]

Since I had obviously lost the spring, I bought a servo air valve repair kit, which included the valve cover with the cap which included the missing spring a new filter and seal, together with a new reaction valve diaphragm. I also purchased a servo seal kit, as it contained a new seal for the air valve, and the gasket that goes between the valve housing and the slave cylinder body.

After it was all fitted I have to say that the power assistance behind the brakes now seems even greater :shock:

Earlier in the week I tried the brakes with the vacuum non-return valve removed, to see how the brakes faired without servo assistance. I could easily tell the difference with and without servo assistance, but now that assistance seems even more!!!

Unfortunately, the original problem with the sticky brakes is still with me I can pretty much make it do it to order now, by either pushing hard on the pedal a few times, or starting the engine with the brake pedal applied. When using the second method I can feel the pedal drop a bit more as I would expect, as the vacuum comes into play.

I read on here, (from Harvey I believe), that with the brakes in the locked state, slacken off the pipe out of the m/cyl. if the brakes free off, the m/cyl is the problem, if they don't, then slacken the out pipe from the servo, and if it frees them off the problem is in the servo.

Using this method, the out pipe from the servo released the brakes, so I guess it is time to remove the servo completely and service the whole unit

Strangely, a couple of times before I tried loosening off the pipes, I fiddled with the pedal height, hoping that it may have been the problem, and the brakes freed. Once I simply pulled on the master cylinder push-rod and they freed, another time I adjusted the pedal height and they freed.

Is there a tried and trusted method of adjusting the push-rod/pedal height properly, so I can be sure that this is not a red herring?

Is it possible that adjusting the pedal effectively pulls on the servo push rod just enough to free the brakes, which is why I had this effect?

 

[harveyp6]

Is there a tried and trusted method of adjusting the push-rod/pedal height properly, so I can be sure that this is not a red herring?

Pull the carpet back in the footwell. Measure directly downwards from the lowest part of the pedal to the floor. If that measurement isn't 6.5" (or 6.875" on a manual) set it so it is.

 

[codekiddie]

Pull the carpet back in the footwell. Measure directly downwards from the lowest part of the pedal to the floor. If that measurement isn't 6.5" (or 6.875" on a manual) set it so it is.

Thank you Harvey. I will check it out tomorrow.

 

[codekiddie]

I checked the brake pedal height this evening and it was too high, so I have now lowered it to the correct height

In which direction does it become a problem with not releasing the brakes, pedal too high or too low? :?

Other chores meant I did not have time to take her for a drive and see if there was any difference, but hopefully tomorrow I can try.

 

[harveyp6]

In which direction does it become a problem with not releasing the brakes, pedal too high or too low? :?

Pedal too high. If the adjustment raises the pedal to the point where it can go no further then continuing to screw the adjuster further in the same direction just moves the m/cyl piston down the bore.

 

[codekiddie]

In which direction does it become a problem with not releasing the brakes, pedal too high or too low? :?

Pedal too high. If the adjustment raises the pedal to the point where it can go no further then continuing to screw the adjuster further in the same direction just moves the m/cyl piston down the bore.

Thanks Harvey, that makes sense. Here's hoping it has had some bearing on my problem :?

 

[codekiddie]

I had a chance to take her out a few times over the weekend, and have experienced no more problems so far, so here's hoping that is the end of it

Thanks to Harvey and Ron for your help so far:- I'm always learning on here

 

[classicalgreen]

just replaced both front callipers with new. held system though very small amount of air I suspect still there as very slightly soggy and can be pumped up to go hard. ( will need trying again later) went to local club meet and car slowed. thought it was fuel starvation ( 22mph) got to meet and parked then notice lots of smoke from both front wheels. checked and well hot.seems brakes applied and didn't 'unapply' though rears seems unaffected .
opened fluid line to servo and resealed brakes then released showing pressure was retained in system for front brakes ( even though a single system) .suspect there may be a differential valve in brake line ?( as in transits) that separates pressure levels to back and front ( no idea if there is or not)
chatted to an 84 year old at the meet whom had those cars before and says its always the servo . seems any amount of dirt etc will cause issues and have brakes lock on and he has seen it before. so new servo next month sigh ( money pit old cars) .
have also fitted new master cylinder just after new callipers as found it was leaking in bulkhead. rear brakes seem ok apart from poor handbrake effect. ( I won't park on a hill even though it passed mot like it)
will update later when servo has been changed ,see if it occurs again. strange but on way back brakes acted normally all the way apart from a half inch of sign feel to pedal which I suspect is a bubble of air in system. will be looking to get that fixed too.

 

[symes]

Not being funny -but have you changed the front flexi hoses? had this a few months ago on a 2.2--the hose inside has caved in allowing a one way action as far as brake fluid went---might not be your problem--but worth a check