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?

[bkrantz]

Thanks for all the replies.

Yes, I can lock up the fronts. It takes a good shove, and maybe the left locks before the right.

I hear you about bleeding and stubborn air. Like I said, I went through several episodes with different techniques. And I will do more, with all the chanting and banging on components I can think of.

I went through the bedding process again today, and maybe the brakes feel a bit better.

And I realize that 1960s engineering, and small, single piston calipers can only do so much. But I still think I should feel more bite and urgency when I get on the brakes hard.

[Superhawk996]

But I still think I should feel more bite and urgency when I get on the brakes hard.

OK - so we know there isn’t a major defect or massive air in the system if you can lock the brakes.

The desire for more “bite” is a fair feeling to have. However, what we do know for sure is that you can generate lock but you don’t like the effort (pedal force) required to do it.

The problem will be trying to tune a system via the internet.

My first recommendation would actually be to install the Tangerine reinforcement to make sure as much force gets to the master cylinder as possible.

My second would be to recognize that you went to the 19mm master cylinder which is working counter to what you want to achieve - which is having less pedal force get you to an equivalent line pressure. The problem here is 1) finding a 17mm master 2) reopening the hydraulics to install it and having to redo the bleed.

Finally, I suspect you are indeed expecting a manual brake system to perform like a power assist brake system we all have become so used to. A manual brake system is largely linear. More pedal pressure is needed to generate more brake output pressure.

Vacuum assist and modern electronic brake boost systems can provide non-linear output that cannot be matched by a manual brake systems. I’ll find a graph to show what I’m talking about.

Note how a vacuum booster initially has lots of output at the front of the curve (green and magenta regions) but then tapers off (blue) - I suspect this lis the sort of “bite” you’re looking for.



Look at the yellow region in the bottom left- there is almost a vertical region in the performance curve. Very little increase in pedal force is getting a lot of output. This “jump in” can’t be duplicated by manual brakes.