Break pedal fade

[Deenracing]

We have a 5-16 with J-mar disk break set up front and rear. Rear is reg stub not micro. We replaced the VW master cylinder with 81’ vw rabbit truck 22.8mm bore. Oddly enough bolted right up. When we bleed the breaks they work great lock up all 4. We can stand on the pedal after bleeding and get no pedal drop. While driving in the rough we loose pedal and have to pump it to get it to come up, but only in the rough. Graded roads, pavement sections, no problem. Only in the rough. We’ve contacted J-mar they gave us a few suggestions but have come up short. Looking for any theories or problematic issues others have experienced. Feel free to assume nothing but we feel we’ve gone over just about everything, new high quality break lines, etc. any help is greatly appreciated.

Thanks in advance.


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[5racer]

call Jamar i a
m sure they can help

 

[J Burleson]

Sounds like caliper “knock back”. Every bit of minor flex of caliper mounting bracking, caliper, rotor, hub, “knocks”the caliper pistons back in the bore, and away from the rotor. Now you’ll have to pump the brakes to get the pistons back out to the rotor face. You don’t see it happening on smooth sections because the is less load to flex or deflect those parts. A 2lb residual pressure valve will help a little bit, but most likely not enough. I’ve even seen bad knock back on the street when big tires are installed on parts that weren’t designed to handle the much larger load forces.
I guess the first question is, how beefy are your caliper brackets? I would also assume that the stock stub axle is gonna flex quite a bit more than a micro stub, so that could also be an issue. Tire size?

 

[JWHracing]

I just dealt with a similar issue on my baja. After hitting a rough rocky section the pedal would go to the floor on the first push. Then the second would be right at the top of travel where it should be. I bought parts to put residual pressure valves in line with the brakes. I am running a late model type 1 dual circuit master probably similar to yours. I installed a 10 lb residual valve in the line running to the rear brakes (I still have drums) and was able to eliminate the problem. Pedal is right at the top no matter how rough it gets.

I bought all Earl's fittings to convert from 10mm bubble flare to 3/16 SAE line. I also replaced the stock rubber lines at the trailing arm pivot with stainless braided -3an lines. Brakes work as they should now.

 

[Deenracing]

We have residual valves already. We may go up in size. We have the brackets that come with the kit. The “flex” is something we’re already looking into. But in all the other cars and ppl I’ve talked too the brackets are fine. Also running 33” Yokohama’s.

Thanks guys keep the ideas coming. This issue is extremely frustrating.


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[jon coleman]

We have residual valves already. We may go up in size. We have the brackets that come with the kit. The “flex” is something we’re already looking into. But in all the other cars and ppl I’ve talked too the brackets are fine. Also running 33” Yokohama’s.

Thanks guys keep the ideas coming. This issue is extremely frustrating.


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exsessive wheel bearing end play was a problem i dealt with. in bumps wheels rock back& forth pushing pads in just enough to be annoying.combine it w - a slightly bent rotor and u are $ chasing.so check all the free stuff first.usually thats the ticket.

 

[Bert is my name]

If you have a floor mount master cylinder you probably need residual pressure valves. Rotor runout can cause pad knock. Uneven or over torqued wheel studs can bend brake rotors especially thin ones. Definitely check run out and bearing adjustments first. Check run out with wheel installed if possible. Could be a combination of contributing factors. The pads in theory should not get pushed back if the rotor stays parallel and the caliper is not flexing. Flexing caliper brackets would be difficult to rule out in the shop. The loads will vary with driving conditions.