Sticky brakes

brynburdett · Post by **brynburdett** » Tue Mar 08, 2016 2:16 pm

Hi Guys,

I'm hoping someone can help shed some light on how best to cure a braking problem on my 1971 S3 2+2.

I bought the car last year and whilst not exactly a 'barn find' it hadn't turned a wheel since about 1989 consequently it had a few 'issues', most of which we have worked through and the car now runs beautifully.

Most of the braking system was replaced. Discs, pipes, calipers, hoses, pads. However I noticed that after releasing the pedal the brakes would stick on for a second or two before freeing up. I took it into a local garage who told me that there was a small valve? (would it be in the servo??) which served to balance the brakes and this had been sticking. They took it out, cleaned it and replaced it and the brakes were fine for the 5 mile run home.

Unfortunately next time I drove the car the problem re-occured and got progressively worse until after 30 miles they locked on completely. I called the RAC and by the time the recovery boys arrived they had freed up again.

I obviously need to sort this out once and for all. I'm 'told' that this is a reasonably common problem on the S3 but I can't find anything specific in the forum.

Can anyone help me identify the problem and suggest a permanent cure as it's really frustrating given that most of the braking system has been replaced.

Thanks, David

MarekH · Post by **MarekH** » Tue Mar 08, 2016 3:28 pm

Dear David,

Get yourself a Parts Manual for the car and then we will be able to direct you exactly to each component of the braking sytem, how each bit works and what effect it'd have if it were not working.

I can guess where this is heading but it'd be more useful to deal with agreed defined terms rather than with "small valves" and maybes.

The Repair and Operations Manual also has a comprehensive description of the braking system. Having these two publications also alerts you to non standard components which may have replaced or "upgraded" the originals, of which we know nothing:- there is no valve to "balance the brakes" on the standard car.

kind regards
Marek

christopher storey · Post by **christopher storey** » Tue Mar 08, 2016 5:01 pm

The servo master cylinder very often sticks, and causes precisely this sort of problem . One of the troubles is the lack of lubricity of modern brake fluids. I have dealt with this extensively on threads in the past - try searching " red Rubber grease" and see what turns up. Although my problems have been with 4.2s, the servo is virtually identical

Edit : here is an example

viewtopic.php?t=5795&highlight=red+rubber+grease

Alty Ian · Post by **Alty Ian** » Tue Mar 08, 2016 9:58 pm

Your master cylinder reaction valve is sticking. This will be either caused by a lack of free pedal travel, the brake pedal should have a small amount of free play when it returns, or as Chris has said the reaction valve which is in the end of the master cylinder is sticking.

Start by checking and adjusting the pedal travel. If it persists then remove, clean and lubricate the reaction valve with brake fluid in the end of it the master cylinder.

Common problem easily fixed.

PeterCrespin · Post by **PeterCrespin** » Wed Mar 09, 2016 3:20 am

Alty Ian wrote:Common problem easily fixed.

I would begin by diagnosing the illness, before starting any therapeutic intervention.

1) Disconnect the vacuum source and plug the hose.
2) Drive car to see if problem persists.
3) If not, attend to vaccum fault -probably the reaction valve.
4) If so, trace and fix hydraulic fault.

christopher storey · Post by **christopher storey** » Wed Mar 09, 2016 9:36 am

Ian : my point was not about the reaction valve, but about the secondary shuttle in the servo, which unfortunately is much more difficult to cure !

brynburdett · Post by **brynburdett** » Wed Mar 09, 2016 4:46 pm

Thanks everyone for the advice.

However, given that this valve has been taken out and cleaned once before am I not better to replace the master cylinder completely?

David

Alty Ian · Post by **Alty Ian** » Wed Mar 09, 2016 5:27 pm

As pete says above you need to diagnose the problem first.

Have you checked the brake pedal has free travel yet? easy to do.
Then rule out a vacuum problem

Replacing the master cylinder may not solve it.

brynburdett · Post by **brynburdett** » Wed Mar 09, 2016 5:54 pm

Thanks Ian,

i've just checked the brake pedal and there doesn't appear to be any free play, so i'll start there. I'm beginning to get a clear message which is that if it's the reaction valve in the master cylinder then it should be relatively easy to identify and cure by a process of elimination.

I'll have a bigger problem if it's the secondary shuttle valve which Christopher mentioned. Christopher, in your opinion would that require a new servo?

David

christopher storey · Post by **christopher storey** » Wed Mar 09, 2016 6:58 pm

David : not usually - with each of the cases I have dealt with ( about 3 in all ) the shuttle and its seals were in good condition, as was the servo chamber diaphragm , and all that was necessary was thoroughly to lubricate the seals with plenty of red rubber grease and reassemble. What is not made clear in the workshop manuals is that to some extent the return of the shuttle to the non-pressurised position is dependent on the large return spring in the servo diaphragm chamber . This is in fact only a very light spring ( though of large diameter ) and it does not take much "stiction" in the shuttle cylinder to overcome that spring and thereby result in the brakes sticking on. In the first instance that I encountered the problem , on my brand new rebuild of my FHC , the brakes stuck on for 24 hours !

It is probably worthwhile trying dissolving a good quantity of red grease in fluid and flushing the system through with it before going to the trouble of dismantling the servo unit and cylinder

brynburdett · Post by **brynburdett** » Wed Mar 09, 2016 8:38 pm

Thanks Christopher, sage advice

David

Alty Ian · Post by **Alty Ian** » Wed Mar 09, 2016 8:50 pm

When I had the same problem, I followed Chris's advice and my brand new shuttle was a bit sticky, however, the main problem was a lack of free play in the pedal, as little as 2mm too little play will cause the symptoms you have.

Mine worked perfectly the moment the pedal fully returned and has done ever since.

christopher storey · Post by **christopher storey** » Thu Mar 10, 2016 8:09 am

Ian : that's a very interesting observation. I have never checked my free play ! Sorry to ask a daft question, but where do you measure it and what should it be ?

Alty Ian · Post by **Alty Ian** » Thu Mar 10, 2016 3:21 pm

Lift the brake pedal up and then depress it while watching the plunger where it enters the master cylinder, there should be some slight free play. If there isnt that means when you release the pedal the reaction valve doesnt fully retract and it holds pressure in the end of the master cylinder.

Thats exactly what I did, as soon as I adjusted the pedal and there was free play everything worked perfectly when moments before the brakes were locked on. Never had a problem since.

42south · Post by **42south** » Fri Mar 11, 2016 11:17 pm

Hi guys,
I agree with the need to diagnose the problem, before jumping to conclusions.
If your diagnosis points to the reaction valve on the front of the master cylinder then I would add one comment to the suggested fix of red rubber grease. When you have the little piston out, take a small piece of grit 2000 wet and dry sandpaper and polish the bore the piston operates in.
I went down the track of grease twice with a brand new M/C, but the stickyness would return after a period. Polishing the bore fixed it for good.

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