The Stovebolt Site Forums The Shop Area The HiPo Shop Rear disc brakes...worth the effort?

I'm running an '84 corvett front end with disc brakes. The rear brakes are '86 Camaro drums. As I have a pedal through the floor set up, it means a long pedal throw before the brakes grab. Due to limited space I run a dual booster under floor to get all the braking I can which isn't enough as I don't think I can lock up the wheels. I would like to know if you think converting the drums to disc would make that much difference in total braking?
Another question is...is there a way to change the pedal ratio? If I could figure out how to get a longer throw of the pedal below the pivot point that would allow me to decease the pedal ratio and move the brake grab point higher on the pedal. I see no way to move the pivot point up to decease the ratio. Thanks

The brakes should be able to be locked up, even with a Corvette disc/drum master w/o boost.

Ed

I agree with Ed, you should have no problem stopping with what you have. Not seeing the set up, it's hard to tell what you have going on. Pedal ratio might be easy enough to change, but then we can see what you have now. Power and manual brakes have different ratio's, so maybe you some how got the wrong one.

Try adjusting the rear drums up tight, then see if the pedal is tight or about the same, if its tight, your self adjusters are not working or you need some residual valves in line to hold a couple pounds of pressure on the rear brakes. If the pedal is still soft, you have other problems.

Joe

It sounds like your master cylinder has too small a bore diameter. You need to move enough fluid to get the pads and shoes tight against the drums and rotors. Try going to a M/C about 1/16" or 1/8" larger diameter and see if you get a shorter pedal stroke.
Jerry

I'm running a Corvette master cylinder now. It's got a 1-18" bore. I did notice the last time I bled the fronts how little fluid came out per pedal stroke. My old Ford pickup with disc will shoot a stream half way across the room. Maybe I'd better start looking at the residual valve and proportioning valve to see if there is a restriction somewhere. I'll take a look at the flex lines also.
I'll just start at the master cylinder and work toward the wheels.
I know I've got the wrong pedal ratio as car started out with non booster. Pedal through the floor creates a problem that I have yet to figure out a solution to. Thanks.

I have been scratching my head on this issue. I'm a one man show in the shop, and getting up and down is really getting as old as I.
I purchased a cheap vacuum pump last year to get my Zephyr vacuum wipers working. It pulls a steady 15"Hg of vacuum. I'm thinking that if I use it with a catch bottle and timer I can test out the four brake lines and the other inline devices. 15"Hg of vacuum should do it, right?
On another note, a car buddy who owned two brake shops at one time thinks there are three different proportional valves. One for disc/disc, one for disc/drum and another for?. Care to comment on that idea?

One of the purposes of a proportioning valve is to restrict pressure to the front brakes to avoid front wheel lockup on slippery surfaces like mud or snow. The rear brakes only are supposed to apply with light pedal pressure so the vehicle can still be steered- - - -locked-up front wheels tend to plow ahead in whatever direction the suspension is pointed under slippery conditions. It's possible your valve is clogged, or the opening pressure is way too high to the front brakes for some reason. Try bypassing the proportioning valve temporarily and see if the brake performance improves.
Jerry

Is the Corvette master you're using meant for disc/drum or disc/disc?

If you're not getting the brakes locking up with your rear drums, discs won't help the matters. Sounds like you've got to have an expert look at your system.

The proportional valve is a PV-2 which is for disc/drum. Got the brakes bled. The vacuum pump didn't work out for the front for some reason. The front section of the master cylinder may have a blocking valve in the rest position. The proportioning valve was in a failed position because of the blown rear cylinder. I reset it per instructions.
Just used self bleeders with pumping pedal and all seems well except the long throw on the pedal as I described before. Short of a major re-design of the brake pedal setup I'll not be on anyone tail end. I really dread the rush hour traffic in Louisville at the NSRA Nations.
Thanks for the help guys!

Since you are using the under floor master and booster, do you have any residual valves in the system?

Seems in the back of my feeble mind I had to install a 2lb residual valve for the front and a 10lb for the rear (disc/drum) when I was having pedal problems on a 50 Chevy I built.

The residual valves helped a lot.

But maybe others with better ideas will chime in soon.

Yes. 2 lb front and 10 lb rear. I went through the who system and everything is operating as it should. As far as I can tell.
The only ting I didn't mention is I'm using dot 5 fluid.

I don't think that would cause any pedal travel issues.

Is there an adjustment rod between the master cylinder/ booster and the pedal assembly.

One other thought, if running a booster, I had problems with a set up where the booster rod that goes into the master cylinder had to be adjusted to take up some slack between the booster and the master.

All the adjustments have been made in that area. I have about 1" of free travel, the rod between the booster and master cylinder is at 1/16" to 3/32". The rod to the booster from the pedal is adjusted for the 1" of free travel.
My unsolvable problem, at least for me, is the pedal ratio. The length of the pedal from the pivot point to the brake pad is too long in relation to the length of the bracket on the other side of the pivot point that the booster Rod hooks to.
The brake assembly is attached to the frame rail below the toe board. The brake pedal attaches to the pivot on the inside of the rail. The pivot shaft extends through the rail to the outside. The booster Rod clevis attaches to a 1-12" or so bracket with hole for the celvis pin welded to the pivot shaft. The booster Rod has to be a straight shot into the booster so I can not lengthen that bracket. Can not move the pivot point up as there is no where to put it as this area has a lot of stuff around it.

Take that 1" of travel out, all you need is enough free travel so the master cylinder is not engaged. The pedal should engage the master cylinder as soon as you step on it, even if it goes through a booster.

I did that several years ago before going on a Rod run from the Cherokee Indian Resevation up the Blue Ridge. Made it as far as Pigeon Forge (25 miles) and brakes froze up due to heat. So, is the heat thing going to bite me in the arse again? I'd love to move the brake grab point another inch off the floor. Please advise. Jerry??. Thanks

I'd reduce that 1" free pedal down to 1/2" or so. Any extra stroke you can get on the M/C is going to be multiplied by the hydraulics before it gets to the wheel cylinders/caliper pistons, so a small change in effective pedal stroke should make a big difference. The lack of volume at the front bleeders is a big red flag, so I'd recommend trying to solve that problem first.
Jerry

I believe now, after bleeding them with the pedal, there was a certain amount of operator error involved. For one, I was using too large a bleeder hose to the catch can. Kept getting air and was always unsure where it was coming from. The second, when using the pedal I got a reasonable amounts of fluid in catch can.
I'll reduce the free play and give it a test drive.
Thanks.