I just a set of Wilwood disc brakes on the rear of my 1968 LeMans drag car. The master cylinder is pitted and beyond rebuilding. I used a master cylinder off of a manual front disc "A" body car when I changed the front to Wilwood discs.
Since I am going to replace the master cylinder, I am considering using a master cylinder from a 79 4 wheel disc "F" body car with manual disc brakes. The manual cylinder has a 1" bore. The power assisted cylinder has a 1.25" bore. It is very difficult to locate a manual cylinder, only few places even list a replacement. Autozone & NAPA have one in stock for power assisted discs. I am willing to try the larger bore cylinder only because the calipers have 4 pistons instead of the one like the stock calipers use. What do you think? Will the pedal effort be too great? Lots of adrenaline at the finish line, I don't think I'll have a problem.....

Big block? Small Block? Nope, It's a Pontiac Engine!

I just a set of Wilwood disc brakes on the rear of my 1968 LeMans drag car. The master cylinder is pitted and beyond rebuilding. I used a master cylinder off of a manual front disc "A" body car when I changed the front to Wilwood discs.
Since I am going to replace the master cylinder, I am considering using a master cylinder from a 79 4 wheel disc "F" body car with manual disc brakes. The manual cylinder has a 1" bore. The power assisted cylinder has a 1.25" bore. It is very difficult to locate a manual cylinder, only few places even list a replacement. Autozone & NAPA have one in stock for power assisted discs. I am willing to try the larger bore cylinder only because the calipers have 4 pistons instead of the one like the stock calipers use. What do you think? Will the pedal effort be too great? Lots of adrenaline at the finish line, I don't think I'll have a problem.....

Big block? Small Block? Nope, It's a Pontiac Engine!

The master cylinder with the bigger bore will give you more brake pressure but maybe not more pedal effort. There is a formula to figure it out but I don't remember what it is.

A 1" or 1 1/16" bore master would be ideal. I'm running a manual brake master off an older Corvette. If all else fails, Wilwood does sell their own 1 1/16" bore master cylinder that might bolt up.

The larger the piston in the master cylinder, the higher the pedal effort will be.

Forget all of that and go with the big bore ALUMINUM unit from Mopar. Found on some trucks and mini vans, you will not have a problem finding one.

After you get it, you will realize that the Mopar mounting studs on the firewall are about 1/8" narrower than the GM mounting studs. You will then either take a drill bit to them (be carefull not to take too much!) or put the cylinder in a end mill and remove some of the material.

"I ain't a killa but don't push me....."

Tupac

79-80 4WD same 1" master as the power front disc just no residual P valve in rear line.Often that is just pressed on the flair fitting. The 81s had the 1 1/8" quick take up aluminum.

Larger bore may be need to move the volume of the rear pistons in comparison to the samller drum wheel cylinders. It also wil give less pessure out with same input pedal in because it is moving more volume.

I used a stock front disc, rear drum manual master cylinder w/1.25" bore. Installed the clevis pin in the highest hole in the brake pedal assembly. Set the adjustable proportioning valve to feed 55% pressure to the front discs. Car will now stop from 125 MPH fast and easy enough to make the first return road with ease! http://forums.performanceyears.com/g...on_biggrin.gif Yessss!

Big block? Small Block? Nope, It's a Pontiac Engine!

On my A-body I used gm metric car front brakes and spindles and on the rear I used metric calipers with jeep wrangles rotors and caliper mounts from the circle track world. For the master I got a reman early 80's S-10 unit with manual disc/drum. Its also aluminum and was less than 20 bucks at the local parts store. The system works much better than the old drum setup and I'm not trying to rip the steering wheel off at the end of the quarter anymore.

Brake effort will increase 56% going from a 1" bore to a 1.25" bore. (100 lbs increases to 156 lbs)

That may be, but I can tell you that drum brakes cause at least a 56% increase in the "pucker factor" over the disc brakes I am now running! http://forums.performanceyears.com/g...s/icon_eek.gif

Big block? Small Block? Nope, It's a Pontiac Engine!