Front disc brakes locking up

[John Milner]

My front disc brakes started locking up on my 68 charger. The front discs are single piston floating calipers. They are new and the lines are new. The master cylinder is a stock one for manual drums. Could this be the issue?

[c00nhunterjoe]

Locking up as in skidding, or locking up as in they are stuck? If its skididng, did you change the drum distribution block with a disc proportion valve?

[John Milner]

They aren't skidding, just holding pressure it seems like. For instance, when I am approaching a stop sign I can let off of the gas and it feels like I'm pressing the brakes. The car will still drive, the front calipers just don't seem to be releasing. I just got through restoring the car so it doesn't have many miles on all of the new parts. I didn't use a disc proportioning valve. I just used 73 single piston disc brakes, new stock 68 master cylinder and all new lines. I wasn't sure if maybe the drum master cylinder has a residual pressure valve built in it or something that might not let the caliper fully release?

[chargerbr549]

On a drum brake master cylinder you will need to remove the residual pressure check valve for the front disc brakes (thats what I did) or just install a disc brake master cylinder that fits your application, that would be one of the first things I would try.

[John Milner]

I ordered a master cylinder that is for manual disc brakes (1976 Plymouth Volare).  I'll start there and see if that fixes the problem. 

[b5blue]

You are still going to need to reduce pressure to the rears. https://www.allpar.com/history/mopar/brakes/disc-main.html

[John Milner]

The Volare manual disc master cylinder did the trick. It stops as it should now.

[b5blue]

 

[c00nhunterjoe]

Fwiw, i run disc/drum with the drum distribution block and have no problems on the street or track with proportion.

[John Milner]

Fwiw, i run disc/drum with the drum distribution block and have no problems on the street or track with proportion.

Good to know.  I drove it quite a bit last weekend with the new master cylinder and it seemed to do well with just running the drum distribution block. We'll see.

[b5blue]

   I added a NOS Mopar Performance bias adjuster. Made for track racing it's a big brass affair with a large green knob. From the instructions it was intended to be installed so the knob could be turned at the dash to compensate while racing. I actually need to go out in a light rain an re test bias as I've put about 3K on a front disk upgrade and brake overhaul. (Per Disc-O-Tech outline.)

[chargerbr549]

Another way to adjust the front to rear bias besides using an adjustable proportioning valve is if the rears are wanting to lockup before the fronts you can go to a mid 70's half ton dodge pickup and get the smaller 7/8 wheel cylinders part #WC37236 which is what I did on my 69 Charger, I believe the factory rear wheel cylinder size is 15/16's. Or if your fronts are locking up you can go to the 2.60 bore front calipers I believe were on the 73-75 A bodies instead of the more common 2.750 bore calipers. Doing this may or may not be enough to help your bias but it worked for me.