Spongy brake pedal after Changing brakes

[91167coupe]

Hi all
On my 67 912; I have changed full brake system, old NOS(I Think) 19mm ATE dual circuit pump, New lines and hoses, Slightly used vented caliper in front and overhauled wide L in rear. And a new BMW ATE dual reservoar at same location as singel circuit res. I have aired the system several times but pedal still very spongy? I dont think it get any harder when pumped. Cars on stands so have not tried the brakes in real.

Anyone know what could be wrong? Some air left some place? Something with the Main pump?

[gled49]

Drive the car, carefully if needed, bed the brakes, might take 50 miles, then assess your brakes. Do a final bleed if needed.

[Jim Breazeale]

Drive the car, carefully if needed, bed the brakes, might take 50 miles, then assess your brakes. Do a final bleed if needed.

+1 on Gled’s recommendation. As you are driving you may want to keep lightly pumping the brakes to keep some pedal feel. Your brakes will magically heal themselves.
Been there, done that many times.

Jim

[91167coupe]

Thanks Gled49 and Jim, I will do that!

[Arne]

One additional thought - I had this issue on my '72T with some new generic no-name pads, but no amount of driving and bedding them in made them any better. I eventually replaced them with name-brand premium pads (Pagid) and the problem was instantly cured. So if your pads are not one of the good known brands and don't respond to good break-in and bedding, you may want to try a different set.

[raspritz]

A '67 912 has extremely problematic high points in the brake lines that trap air on a full brake system rebuild. It typically takes quite a few rebleeds and an extraordinary amount of imaginative cursing to get the brakes working properly again. Get the car off the jackstands and bounce the car up and down on the shocks, one corner at a time. Then jack up each corner very high one at a time and bleed. Remember proper order is RR>LR>RF>LF. Maybe do that process several times. Be prepared for a royal PIA.

[John912]

When you say pump I assume you mean master cylinder? In any case, when you went to dual circuit you had to modify the plunger rod that engages the master cylinder. There should only be 1mm clearance till it engages the piston inside the MS -- check that clearance.

[ejboyd5]

Perhaps hysteresis is the culprit. Not to be overly technical, but it is possible that the memory of the rubber piston seals is pulling the pistons too far away from the discs when pedal pressure is not being applied. To correct: 1) Remove the new brake pads; 2) Substitute thin sheets of plywood or metal for the pads (thinner that the new brake pads); 3) Apply pedal pressure to fully extend the pistons; and 4) Manually retract (push back) the pistons JUST ENOUGH to allow you to slip the new pads into position. By doing this you will reset the memorized position of the rubber seals relative to the pistons so that they are no longer excessively retracted when the breaking system is at rest.

[91167coupe]

Thanks Rick, John and eboyd5, I will check all these things. Yes, I put on a correct rod (I think), will double check clearance.

One more thing regarding brake switch. Is the small 1-pole switch on the dual pump for brake lights? The original one has 2-poles?

[Jim Breazeale]

Perhaps hysteresis is the culprit. Not to be overly technical, but it is possible that the memory of the rubber piston seals is pulling the pistons too far away from the discs when pedal pressure is not being applied. To correct: 1) Remove the new brake pads; 2) Substitute thin sheets of plywood or metal for the pads (thinner that the new brake pads); 3) Apply pedal pressure to fully extend the pistons; and 4) Manually retract (push back) the pistons JUST ENOUGH to allow you to slip the new pads into position. By doing this you will reset the memorized position of the rubber seals relative to the pistons so that they are no longer excessively retracted when the breaking system is at rest.

Good advice, but just driving the car and using the brakes does exactly the same thing.

Regards

Jim