gta rear brakes binding on

Derby calling !
Try pumping the brake with a rear nipple open and see what happens ,pedal still hard ?dribble of fluid or big spurt ? Sounds like a blockage or failed balance thing .
Master is tandem with one feed to the rear closest to pedal .Fronts are fed via two separate outlets .
That's it so it can't be anything major ...just something stupid like when people put calipers on upside down



Does it feel the same with and without servo assist ?

Okay, a quick update.

I've got the whole car off the ground now with the handbrake is disconnected.

In fact there is some movement at the pedal, probably about 30mm. When the brake pedal is pushed all the wheels lock up tight.

Release and the wheels free off straight away. It's just that the rears stay a bit tight to turn.
As a result is it fair to assume the balance valve at the back is ok as the pressure does seem to release.

Pedal return is slow. I undid the brake servo pipe at the engine and sucked on the pipe to the servo and you can, with some effort, suck air through.

It is sounding like Jon C could be right about the servo being bust. The combination of slow pedal return and air leak.

Would the servo stop the master cylinder releasing the rears fully?

Next plan is to ease off the bleed nipples on the rear calipers and let some pressure out there to see if the rears release properly.

Would the servo stop the master cylinder releasing the rears fully?

What I think is actually happening is that the fact that the pedal is not returning to its rest position means that the master cylinder is also not able to return fully to rest, and as a consequence, it is keeping the pressure in the brake circuit. The master cylinder piston must be able to return fully otherwise the hole, that allows the fluid into the master cylinder from the reservoir, is not uncovered, therefore as the brakes wear the cylinder is not being replenished with fluid. There is a very accurate measurement that must be checked on the servo when it is at rest, so that the operating rod just touches the piston of the cylinder and no more, that is so the cylinder is replenished (even by the minutest amount) each time the brakes are applied.
Below is a pic of the servo where you can see the large conical spring running about 35 degrees (up & down) from the left to right pushing the diaphram back to the rest position. This spring holds the diaphram against the vacuum until the brake is pressed and the valve, which is connected to the pedal, then allows air in, in proportion to how much pressure is applied to the pedal, the rod "P" then presses on the piston of the cylinder applying the brakes. When the pedal is released,the air is exhausted by the spring which returns the diaphram to rest to await the next pedal application.

Fronts are fine

Are these the rears that you rebuilt or new ones ?

Rupert, before you go any further, it would be wise to check the pedal its self for any restriction by say, siezed pivot, bent pivot shaft or foreign body that might be causing a problem. I suggest detaching the servo operating rod from the pedal by removing the clevis pin to check this. I did suggest this some months ago when you started on this, but you did not say if you did this check. Its a long shot, but you don`t want to start dismantling the car only to find out that there was a simple explanation for the problem.

Yes John, I've already undone the pedal from the rod and it is completely free. At rest it sits further back than the rod will come.

Lee, the calipers were recon ones that Richard Walsh regularly uses. still basicllay new.

I undid the bleed nipple on one of the rear calipers and tried the pedal. Fluid came out ok. Pedal travel is better of course with the nipple open and the poor pedal return is very noticable and squeaky/noisy.

The pistons on the caliper screw in fine with the nipple open, but not with it shut.

My guess at the mo is that the servo is not fully returning because it is knackered, so the master cylinder is not fully returning and this leaves the brake pipe to the rears under pressure.

It's either the servo or the balance valve not letting fluid return. But the problem with the pedal return suggests the servo.

Does anyone have a servo? Are there new ones available?

[BIG_MVS]

Does anyone have a servo? Are there new ones available?

I'm afraid not, and you cannot buy them new anymore

Dell Boy was going to see if the 610 was similar/the same as he has those in stock I believe.

I would try the balance return first as I think they are available from renault and I have also seen them on the bay.

Maybe I can get it reconditioned.

Last thing I need for now is a part no for the balance valve.

Fuel tank is out, I'm going in!

If all else fails, it appears the Simon Autos have the Servos in stock. The part No. from them for the Atmo is 131060...... now sit down while I tell you the price.... 367.71 Euros , and the limiter valve part No. is 131064....and again stay sitting down....141.61 Euros .
In all fairness, at this time, I don`t suspect the limiter. I cannot see that the limiter can cause the pedal not to return, as this to me is the deciding factor.....but as always, I could be wrong, but I would replace the Servo going on the information you have given us. Pic below shows all the Brake parts with their Simon Auto Part Numbers.
PS. If you do go down the path of getting the Servo from Simon Auto`s. it would be prudent to change the Check Valve as a matter of course, Pt No. 131089...this time only 13.69 Euros



As a matter of interest, the part No. for the Turbo Servo is different, as is the Pt No. for the 610. Can`t think why

To check if the valve is at fault make up a small pipe to bypass the valve, bleed the brakes and test again. If the fault has goes then the valve is the problem.

You will also need a T piece as there is one pipe into the valve and two out from memory.

Dell Boy was going to see if the 610 was similar/the same as he has those in stock I believe.

Fot those that are interested in this type of useless info, the D500 has one part No,...the D501/2 has another....and the D503 RHD has another according to Renaults Parts List. This is confirmed by cross checking with Simon Autos parts list, but it does appear that all three types are available from them, prices... D500 = 367.71 euros, D501/2 are the same price, but the D503 = 248.71 euros...lucky D503 owners

To check if the valve is at fault make up a small pipe to bypass the valve, bleed the brakes and test again. If the fault has goes then the valve is the problem.

You will also need a T piece as there is one pipe into the valve and two out from memory.

I have removed this pressure limiting/proportioning valve from my GTA and found the braking much better, with the fronts still locking up first if pushed hard.

[quote="rupert"]The pistons on the caliper screw in fine with the nipple open, but not with it shut.

My guess at the mo is that the servo is not fully returning because it is knackered, so the master cylinder is not fully returning and this leaves the brake pipe to the rears under pressure.

quote]
Don't know if this has already been mentioned.
Perhaps the rear brake hoses have collapsed inside meaning that the fluid will go through under pedal pressure but unhappy to go back to the master cylinder when released.

Goodridge have a full set of braided hoses under 50 Pounds and are a good addition anyway making the pedal much more sensitive and have less pedal travel.

Goodridge pipes fitted already Alan.

No one has new servos it seems.

Replacement servo obtained from our leader, not perfect, but a lot better than the old one. I measured the projection of the rod that goes into the master cylinder and you can see that the old one was sticking out about 12mm whilst the new one is about 9mm.

Hoping that Jon was right and that this was the problem.

Old one:



New (old) one: