My C5 front brakes does not work

[RamAir]

I have a -68 Firebird with AFX spindels. The front brakes are 325 mm C5 and rear brakes are original GM drum brakes.
The brake booster/Master cylinder are provided by DSE (GM S10, 1"). All components are new except for the drum brakes.
Unfortunately the front brakes does not work as it should. I have verified that their is no air in the system. I have also mesured the hydraulic pressure in the system. Out from the Master cylinder it is 1200 psi from both outlets. The pressure in the front caliper (measured at the bleed holes) is 1100-1200 psi with 15 inHG intake pressure. Finally 550-600 psi at the wheel cylinder in rear (1100 psi without pressure regulator). So every thing looks ok but ...... When I hit the pedal I lock the rear wheels ! I have very low brake force in the front brakes. First of all, I have replaced the master cylinder, brake booster, pads and it did not change anything. Then I adjusted the rear brakes so they will apply somewhat later just to verify the efficacy of the front brakes. Now it was even hard to stop the car.

Could anyone help me with this ?
Magnus

[Skip Fix]

Have you bedded(broken in) in the front brakes? If not it can take a while for them to start working correctly.

[RamAir]

I did try to do that but since it mostly the rear brakes that stops the car it is hard to the the bed-in process. I wish that I had a force sensor so it was possible to mesure the force from the caliper. I thought that it was impossible to assemble the caliper and pads wrong but I don´t know. My selfconfidens has gone since I have struggle with this for a long time now. I have several track days booked but it is impossble to drive the car as it is now. :help:

[Cdog]

When I was going over my system for my 67 Camaro I worked with a tech at Stop Tech. They are very knowledgeable guys over there. I’d give them a call and buy a set of their pads for the free advise.

[Quickboat]

So if you remove all of the rear brake pressure with the PV, its as if there are no brakes whatsoever?

[chevelletiger]

tobin at kore3 would be some to call also.great guy.

[RamAir]

I can only reduce the rear pressure to 500 psi but i have adjust the brake shoes so they will apply later when I push the pedal. I think there is some force from the front brakes but it is very small, measurement would help but I don't know how.

[Cdog]

Is your plumbing backwards at the master cylinder? I could be wrong with this particular MC but it looks like you have the front plumbed to the rear in the MC and the rear plumbed to the front. Only the rear should have a Prop valve.

[RamAir]

I have spooken to Tobin and he has been very helpful, unfortunately we have not found the problem.

[RamAir]

Good point about the plumbing. The MC that DSE shipped together with the BB was a Buick 80-something were the front port goes to the rear and rear port goes to front. I replaced this unit with a 2000 GM S10 RHD unit and asumed that the configuration would be the same, but I am not sure. Any one familiar with this MC?

[RamAir]

But, the pressure out from the MC are equal and the rear chamber would be pressurized first since it is closest to the BB, or am a wrong?

[Cdog]

But, the pressure out from the MC are equal and the rear chamber would be pressurized first since it is closest to the BB, or am a wrong?


I’ve never in all my years working on cars seen a MC with the front port feed the rear brakes. I would bet that is your problem.

[Apogee]

...First of all, I have replaced the master cylinder, brake booster, pads and it did not change anything...

What pads did you try other than the ACDelco's supplied with your kit? Mostly just curious since your problem sounds friction related, but doesn't seem to respond to the usual fixes.

Have you confirmed that your calipers slide freely in the pad abutment brackets? I'm sure you have, but I'm at as much of a loss as you are on this one given all of the things you've done and tried up to this point. If you're getting 1100+ psi at the front calipers, that should be more than enough to achieve lockup in most applications. The only other moving component up front would be the caliper pistons...do they slide freely in their bores? Generally you would only have an issue with one piston at a time, and none on a brand new caliper, but since we're grasping at straws, I thought I'd throw it out there.

The MC appears to be plumbed correctly...lots of applications have the forward port feeding the rear brakes, including the S10's.

Tobin
KORE3

[CarlC]

Tobin, he's running rear shoes/drums. Is there an unbalance at work here, C5 caliper + stock rear drums?

[Cdog]

What pads did you try other than the ACDelco's supplied with your kit? Mostly just curious since your problem sounds friction related, but doesn't seem to respond to the usual fixes.

Have you confirmed that your calipers slide freely in the pad abutment brackets? I'm sure you have, but I'm at as much of a loss as you are on this one given all of the things you've done and tried up to this point. If you're getting 1100+ psi at the front calipers, that should be more than enough to achieve lockup in most applications. The only other moving component up front would be the caliper pistons...do they slide freely in their bores? Generally you would only have an issue with one piston at a time, and none on a brand new caliper, but since we're grasping at straws, I thought I'd throw it out there.

The MC appears to be plumbed correctly...lots of applications have the forward port feeding the rear brakes, including the S10's.

Tobin
KORE3

Interesting. I have never seen that before on any of my vehicles. You would definitely be in the know.

[72BBSwinger]

I had a similar issue when I converted to Viper front brakes, and yes I even still have rear 11" drums too. I noticed my rotor on the passenger side only had wear on the inside 1" of braking surface. Got under the car and the inside of the drivers side rotor was doing the same thing, pulled the pads and 2 of them were not ground flat at the pad surfaces! Like .060 difference from the top of the pad to the bottom! Swapped in a set of Hawk HP+ pads and now I have Viper front brakes! Check your pads, the ones that came with my calipers were generic junk.

[RamAir]

I replaced the AC Delco pads with Raybestos PGD731M. I will take a day off from work tomorrow and dig deeper in to this, I just have to solve it. Since it has to do something with friction or movement of the caliper I will focus on that, I wish I hade a clamp force sensor, that would help.

[RamAir]

Look at the picture, is it assembled correctly? If a put a pice of wood between the caliper and the piston, should it marks on the wood?