Brakes don't Bite Options

[bkrantz]

As part of my rebuild I completely replaced the brake system:
calipers and rear pressure valve rebuilt by PMB
new ATE 19mm master cylinder
new hard and flex lines
new disks and Porterfield R4-S pads

I went through many (many!) episodes of bleeding, with different methods. The pedal feels very firm from the first push, and does not pump up. Rear clearance is set at .004". I went through the PMB bedding process, and have driven around a bit.

But the braking force seems weak. When I get on the brake pedal, I do not feel any "bite". Another 914 friend drove the car and also agrees.

Any suggestions?

[ChrisFoley]

As part of my rebuild I completely replaced the brake system:
calipers and rear pressure valve rebuilt by PMB
new ATE 19mm master cylinder
new hard and flex lines
new disks and Porterfield R4-S pads

I went through many (many!) episodes of bleeding, with different methods. The pedal feels very firm from the first push, and does not pump up. Rear clearance is set at .004". I went through the PMB bedding process, and have driven around a bit.

But the braking force seems weak. When I get on the brake pedal, I do not feel any "bite". Another 914 friend drove the car and also agrees.

Any suggestions?

Two suggestions:
Add a master cylinder brace to prevent floorboard flex.
Adjust the rear brake regulator to begin regulating at a higher pressure.

[Superhawk996]

Adjust the rear brake regulator to begin regulating at a higher pressure.

Bad plan

The regulator / proportioning valve doesn’t affect initial bite as brake line pressure is ramping up on moderate or even on hard brake brake applications.

The regulator only limits pressure to the rear brakes after pressure has built to the cutoff or knee point. After that point it continues to build pressure to the rear brakes but at a lesser rate than the front. By the time the regulator comes into play (on street use), weight has been transferred to the front axle and the front brakes are doing the vast majority of the work.

The regulator is there and adjusted to prevent lock up of the rear brakes and the inherent vehicle instability that comes with rear brake lock.

Messing with rear brake bias is fine for racing application but shouldn’t be taken lightly for street applications.

Here is the data on how the rear brake pressure is regulated:


48 kg/cm2 is 683 psi - that is significant brake line pressure before any reduction in rear brake pressure build rate even begins.