Please tell me its not the Servo

[nigel bickle]

I recently bought a Renault 25 v6 turbo-sadly intent on robbing it for its engine - for my GTA. However- it seems a rather nice car-and I've found an alternate engine for said Renault- so I'm going to try to save bothAll's well on the 25 except the brakes -seemingly Ok when driven into store 7 years ago ( but they would say that-wouldnt they..).Near solid pedal engine off, little to no resistence - engine running. Replaced all the fluid, (dirty-but not excessively so -no strange rubber bits either) new master cylinder (same as 2 litre), still no change. Servo looks clean/correct-no fluid in it etc. No smoke from exhaust (white/blue/black/grey) -engine tone doesn't appreciably change with pedal pressure.Feels like it should have brakes-but the pedal just sinks to the floor, with minimal 'back'pressure. Switch it off- vrtually solid. pads all sound -less than 25% worn. No leaks, no loss of fluid.Any ideas?If it is the servo - anyone know if its specific to this model/car-or is it interchangeable with ''lowercost'' options?Thoughts appreciated......

[M'coli]

Does it have an ABS pump?

[nigel bickle]

Christ knows -presumably-but surely that would 'chatter' the pedal when is moving? Right now-max speed 3 mph-cos the handbrake will stop that!

[garethj]

Christ knows -presumably-but surely that would 'chatter' the pedal when is moving?

Not necessarily. If the pump is dead then shirely no fluid's going to pass to the slave cylinders. However I'd have thought it would be 'fail safe'?When a brake servo failed on my old XJ-S the brakes still worked, it just drank brake fluid so I reckon your servo is alright.

[nigel bickle]

Cylinders bled normally-when it managed to free the screws off!

[Station]

Does it actually brake efficiently when the engine is running? If the servo has failed, it wouldn't go spongey.Have you tried bleeding the master cylinder (stick a pipe right into the resevoir and suck til there's no bubbles left). I think it's over compensating somewhere - there's a pipe going from the servo to the inlet, try disconnecting this to see if there's any change.

[simmo]

Doesn't sound like servo (to me), could be master cyld. In my experience ABS failure results in a hard pedal and little braking effort and it would fail the self check so the light would come up.Should see the ABS light do something when you start up if it has it. I would clamp off all the flexi hoses and see how that changes things.

[nigel bickle]

New mastercylinder on it-changed nothing.Dashboard is like a bloody space rocket-even the seats have a dozen adjustments each.I've more lights than a Xmas tree -allgo out when its running.M/cylinder is bled out oK- first port of callClamping the flexi's is a damn good idea-thanks- out with the molegrips....

[FredTransit]

Think about it, if you have a hard pedal with the engine off, you have no servo vacuum, so when you have the engine running, if it was the servo it would still be hard. Does that sound right? Is there any fluid going anywhere? Wheels? Into the servo?

[nigel bickle]

Agreed Fred -so whats causing it? pedalishard- engineoff, non existant, engineon. No leaks, no lost fluid, no burning brake fluid via servo. I'm going to try isolating each wheel by clamping flexis- otherwise I'm close to stumped.

[Mr B]

Have you checked the vac pipe from the inlet manifold to the servo and the one way valve?

[mouseflakes]

I was going to suggest one of the flexies is 'ballooning' - I guess that will show up when you clamp them? Although you think this would also show up with the engine off - though there would be more resistance then. Good luck Nige, and well saved!!

[nigel bickle]

Solved-thanks Simmo. O/s rear seems to be the prob. Clamped the flexis individually -immed changed when that one was'shut down'.Full investigation this weekend.Thanks all -it was driving me nuts.

[M'coli]

^^^[fingers crossed]So do the brakes stop the car when that one's clamped off?[/fingers crossed]

[FredTransit]

Ah, now ABS, there be dragons.....

[Station]

I was going to say 'clamp the pipes'! It's the first port of call!

[simmo]

Glad to of assistance, but forgot to say a word of warning some flexis have some sort of liner in them not come across it on a car yet but I did bugger up some PSV ones a while back

[nigel bickle]

Ta. its sorted-bit of crud caught in the caliper. Still a little sticky-but a late night hoon should sort it.Next step -first MOT fail...