Long brake pedal

[ajsphead]

I have a Rover SD1 with a brake problem bad enough to fail MOT, but which has got me and my colleagues foxed.
Engine off - pedal is firm
Turn engine on with foot on brake pedal - pedal drops an inch +.
First couple of applications will still give maximum braking if needed, after that, the pedal hits the end of its travel, you have gentle braking only, and you have to apply the pedal once, then a second time to get a firm pedal and proper braking power.
Turn engine off and the pedal goes firm again.
You can leave the car for days and the pedal stays firm.
You can pump up the brakes, stamp on the brake pedal and hold your foot there for 10 minutes and the pedal doesn't budge.
The system has been bled to death, the m/c seals are new, the rear self (ahem) adjusters are new and lubricated, the front calipers are fully refurbed, and the rear shoes are new.

Thanks

Anthony

[tritonofnor]

If the problem only occurs with the engine running I would suspect the servo..

[Luxobarge]

If the problem only occurs with the engine running I would suspect the servo..

Seconded - I'm not sure what would be wrong with it, but that definately sounds like a servo problem to me.

[JPB]

Thirded!

Check that the servo filter is working properly, this sounds like it isn't breathing as it should.

[suffolkpete]

If it's the servo, I'm struggling to understand why the pedal goes to the floor, because the servo is basically just a rod that pushes on the master cylinder and the hydraulics must be ok otherwise you'd be losing fluid but the master cylinder is going to the end of its travel. It's not clear from your post whether the problem only occurs when the car is moving. If that's the case then something is happening that's causing a lot of travel somewhere, I would suspect the handbrake or self adjusters. Another thought is that the Rover has four-pot calipers. I'm not familiar with the Rover, but when I owned a Princess wedge with a similar system, I seem to recall that there were three bleed nipples on the calipers and bleeding had to be carried out in a particular sequence. If anything went wrong, the master cylinder travel greatly increased and a warning light came on as the braking was concentrated on one circuit. It has to be something obscure because you've done all the normal things for this kind of fault.

[BXBXBXBX]

Hello, the bit about 'you can pump up the brakes' would for me point to :-
1 Air in the system. Sometimes I have found that in difficult cases that pressure bleeding is the only way to shift All the air.
2 Self adjusters. They may be new but I have found that even new ones sometimes are not that good. Not trying to be funny but it has been known that they have been 'wrong sided' and un adjust themselves. You could try adjusting them manually so they have a slight drag and see if that makes a difference.
3 Is there free play at the pedal so the master cylinder can recuperate.
4 Clamp off the hoses, front and then rear see if it makes a difference.

Was there ANY kind of mark in the bore of the master cylinder and had it been honed at any time?

Play in the wheel bearings?
Just a few scattered thoughts.
Best of luck.
Cheers.
Geoff.

[ajsphead]

This is brilliant, thanks to you all. I should fill in a bit more detail. The car is running a Jag XJS front brake conversion. The problem was there before the conversion but the extra effort required to operate the bigger pistons has just tipped the balance over the edge. Regardless of that, the problem still needed sorting, but I only have single line brakes to worry about now.

The problem only occurs with the engine on. It's that initial inch+ of travel when you turn on the engine, rather than just a slight drop which you would expect that starts the problem. You can then use the pedal a couple of times and feel the brakes being appied further and further down the travel until after about 3 applications, then it's gentle braking only on the first touch. Turn the engine off and the pedal is firm again.

Re pump up the pedal. the pedal is firm but with 3 pumps, engine off, it's rock hard.

The self adjusters... Well one is fine, but the o/s/r is adjusted with a nut welded onto the shoe to act as a stop off block. The new adjuster teeth kept slipping on the adjustment point for new shoes. They're fine for adjustment with worn shoes, hence the adaptation.

The master cylinder was an ex India one with new seals, so should be fine.

I was going down the line of a blocked servo filter myself.

Anthony

[BXBXBXBX]

Hi thats making a bit more sense now. The 'pumping up' is the servo using up it's vacuum which is not being replaced because the engine is stopped. The inch+ of travel is the extra force being applied by the servo. As a try, pump the pedal with the engine stopped to exhaust the servo then push on the pedal hard and start the engine the pedal should sink approx. 1 inch, that is the servo working.
The pedal going to the floor more and more with each application seems to point to :-

Originally I thought air but that does not seem so likely now, the bodge with the rear adjuster to be honest may work but is something I would not personally do ,( I dont think it's safe.)no more comment on that I think. To make sure it's not playing about then clamp off the rear hoses. and see what happens.
Just to eliminate any problems with the front then clamp off the front brakes. It's a process of working through things.

If this does not find anything then it seems more and more likely ,and I think it is, that there is either a problem with the free play at the brake/servo/pedal but could be an area of wear in the master cylinder bore so that it is leaking internally.
Basically wash bore with meths or brake/clutch Cleaning fluid and with a good light look for any mark or shadow in the bore. scoring is obviously a no no.
I used to use a very small bulb like a cap-less dash bulb soldered to a length of very thin twin wire and put this down the bore, it is very surprising what it picks out in the way of damage.
Please come back and let us know the results.
Cheers.
Geoff.

[Luxobarge]

Above post has lots of good advice IMHO.

I would stress the usefulness of clamping off flexi hoses as a method of eliminating parts of the braking circuit - if you clamp off something and then you get a good solid pedal, then the fault is the other side of wherever you have clamped. Good advice, especially with a tricky one like this.

Cheers

[mr rusty]

It looks to me like a sevo problem- I think the vacuum valve isn't closing off internally and causing the servo to run continuously so to speak, rather than shutting off when you take your foot off the pedal.....ultimately this will force the m/c piston all the way down the bore past all the fluid, but you wouldn't neccesarily see a leak-shutting off the engine cuts the vacuum and hence the servo assistance...but does the exhaust smoke when the problem happens?

You could try disconnecting the vacuum hose and carefully driving around without a working servo to see if it makes a difference -the brakes will be heavy but still work.If it does turn out to be the servo, Rimmers are knocking these out brand new for £60 at the moment.