Any Brake upgrade tricks

[Original Poster]

Original post.....

[blucille]

ask him to do a quick rebleed, and a recheck.....if the brakes are spongy, it could be new pads need to be bedded in, but after our run up the hudson, they would have felt better.....no reason why he shouldn't recheck his work for you....

[John912]

I'm sure he knows, but don't forget the inside bleeders on the rear calipers -- there are two bleeders on the early rear calipers.

[304065]

What fluid? (Unlikely it could be boiling but if it's old, use SRF)

What pads? Race pads will not stop the car when they are cold, I learned this the hard way one cold morning nearly having to take evasive action in my '88 Carrera on the way to an AX with Porterfield R4S installed. Stock pads for street from that morning forward.

[ibmiked]

Raj,

After I completely disassembled my brake system not only did I have to bleed the brakes twice, but I had to 'cycle' the calipers. Normally when you do a bleed in a shop the pistons come out to meet the rotor but never retract back very far. Air gets caught inside the top of the pistons. I solved this by pushing the pistons back to a fully retracted position after each bleed cycle. This is not something that is done as a normal (read lazy) rule, but made a WORLD of difference. My brakes were spongy after replacing everything in the system and doing the first bleed. I was at a loss until I really thought about it and then did it the above way. They are rock hard now.

You just still have some air in the lines, it's that simple. You could also use the brakes for a while and then do another bleed which should hopefully catch the rest of the air.

Hope this helps,
Mike

[davep]

In addition to what Mike says, sometimes the rubber rings will pull the pistons back too much, so a bit of driving alternated with braking will get the pistons in the best position. Otherwise it may take two pushes to fully apply the pads. This is most evident if pumping the pedal brings a hard pedal instead of the soft pedal.

[JohnT]

Hysteresis!

To quote Wikipedia "Hysteresis is the dependence of a system not only on its current environment but also on its past environment. This dependence arises because the system can be in more than one internal state."

The piston seals are not "O" rings but circular square section rings. These seals (particularly new ones) have enough friction that they will retract the pistons after one steps on the brakes. The solution that usually works is to pump the brakes once (or twice) before the pads are replaced to get the pistons (and subsequently the brake pads) close to the rotor. If the pads drag a bit, it is ok as eventually the system will work out the proper location of the pistons and pads.

Repeated applications of the brakes (as in multiple bleeding) should eventually get the pistons and pads in the proper, retracted, position.

For better performance Porsche did supply vented rotors for the SWB cars. Calipers are the same size but simply have a spacer installed between the halves to make room for the wider rotors.
part numbers are:
901.351.043/4.20 - Large Brake caliper - front
901.352.943/4.20 - Large brake caliper - rear
I have these on #902 (albeit a 912 racecar) and have never experienced brake fade in 10 years of racing.

johnt

[RSTarga]

I only use ATE brake fluid. The -3 stainless lines will harden up the pedal over the rubber lines. I just got one of those motive pressure bleeders and it really gets all the air out.

[C.Plavan]

I would just bleed them again.
In the racing world, I only use the Motive Bleeder for quick after race bleeds if needed. After a full brake rebuild/hoses/etc/ you need a good hefty foot to get all the air out. I tried it with the Motive bleeder, but only a "foot" shoving the brake pedal down would get those last bubbles out.

[EA911]

JohnT and Mike are on the money on this.

I also learned another trick during my install and that is to force the caliper onto the rotors. Gap the pistons+brake pads just a tad narrower than the width of the rotors which will counter the "piston retract" issue. This works - after I re-installed using this method the brake pedal was solid as a rock on one push.

I was concern there might be drag since I forced it in but after a drive around the neighborhood I jacked the car up and checked with no drag issues.

Edric