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?

[gereed75]

Different car but maybe a useful data point - I had some unknown old and what appeared to be metallic very hard pads on my race car with Willword calipers. I called Willwood to get their recommendation for a pad. I gave them car weight, HP, typical track cycles (speeds braking from down to corner speeds how many times per lap and how much time between applications).

I was surprised when they recommended an aggressive street compound. They said my loading was just not enough to get sufficient heat into a race pad to generate good friction. My point is Race oriented compounds need a lot of heat to get into efficient friction ranges. The new ceramic street compounds are very good at generating initial bite with minimal loading while providing good wear characteristics.

Not sure what type of compound the R4S is, but you might be asking it to do something it is not designed for

[Superhawk996]

Not sure what type of compound the R4S is, but you might be asking it to do something it is not designed for

R4S is a street / performance compound so not outright race pads. R4S also have a good history for most but certainly may not be for everyone.

having said that I don’t disagree a bit with prior comments of re-bedding and or consideration of a pad change as a possible improvement - it all depends on driving style.

Each pad has its own friction curve. Some (race pads) have low friction until the get hot. Others have high friction when cold but fall off drastically when hot (prior to actual fade).

One thing that has frustrated me for years in the aftermarket is the unavailability of the curves. They are easily obtained on a brake dyno. The problem is the data is so specific to a particular pad, rotor, caliper combination so the data doesn’t necessarily apply the same between a Nissan and a 914 for example.

For example the Porterfield is listed by PMB as 0.41 mu (friction coefficient) but doesn’t tell you what temperature that is at. No pad has a singular average mu at all temps. This doesn’t reflect on PMB - like I said the pad suppliers just don’t provide good data.

That leaves us with trial and error.

I really used to like Ferodo back in the day but Only God knows what they are like today with all the EPA regulatory restrictions limiting raw materials.

FYI - get ready for things to get worse. Copper is being regulated out of pad formulas for OEMs. It has caused a lot of chaos trying to find new friction compounds that work as well as those that had copper in them. This will eventually trickle down to the aftermarket. If you have a particular pad that you love - stock up now.