Braking problem.

[dougal9887]

Opinions welcomed on this issue....
When braking firmly and releasing rapidly - no problems. When braking gently - releasing slowly the brakes sometimes hang on, a tap on the pedal is required to release them.
Master cylinder, front calipers, pipes and hoses are all new. Rear calipers professionally reconditioned. Girling servo overhauled by me; it was well gummed up but cleaned up ok and new seals and diaphragm fitted.
My first thought was a sticking control piston or it's return spring was weak and that may still be the case. I stripped the servo again and double checked all was correctly assembled. It was hard to judge whether or not the control piston returned smoothly as there's some natural resistance from the seal. The bore looks ok, but that's not very scientific.
My next thought was an air valve not seating properly and allowing uncalled for atmospheric pressure to the diaphragm but all looks ok there and it does pass the test of operation after 2 minutes of engine off.
I don't know if a fault with the new master cylinder would have these symptoms?
I could send the servo for reconditioning, but it's expensive if the fault lies elewhere. Likewise a Lockheed replacement.
I still suspect the servo, but any ideas?
Dougal.

[rgh0]

It could be that the master cylinder is not releasing pressure correctly. This can be caused if the pushrod is fractionally to long. You say you used a new master cylinder. Did you use a new pushrod with it or reuse the old one? From what i have seen the new pushrods are not the correct length on some new master cylinders

cheers
Rohan

[661]

My S4 did similar with new MC and calipers refurbed. It didn't seem to matter whether it was light or heavy braking( that I noticed) , it was sticking intermittently. I suspected the servo, removed it and all sorted. The brakes have never been better. Sure you press harder ( and I have +2 front calipers), but very predictable.

[dougal9887]

Thanks for the replies. Should have mentioned that it's a Plus 2 so I won't be removing the servo.
I used the new master cylinder push rod so I'll have a look at that. Presumably I'll be able to feel if there's a little play before the rod pushes the cylynder, or not, as the case may be?
Dougal.

[rgh0]

Yes the brake pedal return spring should pull the pedal back far enough so there is a little free play and the master cylinder piston is fully released.

cheers
Rohan

[Certified Lotus]

I have learned the hard way to check the length of the pushrod. A slight variance in length causes trouble with the brakes.

[vxah]

My money is on the servo, had exactly the same issue, funny how you learn to "drive around it"
I think you are correct with the sticking control piston although I was unsure of the little "T" piece as it was a little worn so i turned it around when i rebuilt the servo after it leaked all my fluid away. After the rebuild is when the sticking started to happen and i do remember the control piston seal being a little tighter that i would have liked..
I have since had the servo rebuilt by Past parts with a resleeve etc and it does not stick now!

[dougal9887]

Thanks for the information vxah, I'd never heard of Past Parts and their sleeving operation. Classicar told me that if the bores were worn then that was that and a new Lockheed servo was required.
So I'll have a look at the push rod play and if that looks ok then I'll get in touch with Past Parts as I'd prefer to keep the Girling servo.
I also thought the new control piston seal was quite tight, but I'm no expert.
Dougal.

[vxah]

I did not want to fit the Lockheed servo either because it just looks out of place under there?
If we think about that control piston for a moment and understand that it's movement controls the air/vacuum to the diaphragm it makes sense that if it sticks a little when slowly returning it could keep the servo working? Give the brakes a sharpish tap and the speed of the control valves return overcomes the sticking?

Pastparts did tell me at the time that they could not re-sleeve the control valve bore as the did not have sleeves of that size (that may have changed). They did sleeve the main bore and even replated the pipework for about ?230 all done iirc.