I've got a question for the group here. I have a '64 Chevelle that I've done a front disc brake conversion on, and now it does most of the stopping from the rear.

I installed a new disc brack master cylinder, and I used 2000 Firebird rotors and calipers, the calipers are dual piston. The rear axle is a Ford 9 inch with the stock Ford brakes and wheel cylinders.

Now I also installed one of those adjustable pressure valves to try to reduce the amount of pressure going to the rear brakes, however, it's only on the rear line, so I'm not sure exatly how it can work properly if it does not sense the front pressure as well in order to regulate the rear.

Anyway, my theory on this is that because I'm using a stock master for a stock single piston caliper and stock rear drum wheel cylinders, but I'm using a dual piston caliper instead, I'm thinking that there isn't enough volume to build sufficient pressure for the discs, but it's still providing the normal pressure for the drums.

Does this sound right? And if so, is there a master cylinder available that will work with the set up I have?

I can upgrade the rear to disc brakes I guess, but that's expensive, and the rear drums do a great job of stopping the car now. So if the discs were working properly, there isn't really any reason to convert the rear to disc in my mind.

The discs are working to help stop the car, but you can feel how it's pulling the car to a stop from the rear, especially at a slow speed, like stop and go driving. And if I hit the brakes on a dirt road to test my theory, the rears do lock up without the fronts ever doing so.

So any suggestions or comments are welcome. I don't want to be in a situation where really need to slam the brakes and find myself spinning around from locked up rear brakes.

I'm going on a 1100 mile trip in a few weeks with this car, and I don't want any problems.

PLEASE HELP!!!

Your master cylinder is a dual resevoir right? If so, make sure you have the lines correct, the front brakes should go to the rear resovoir, so that the front brakes will apply 1st. Your front brakes are 75% of your stopping power. When I converted my 66 from a single resevoir drum brake sytem, I installed an aftermarket disc brake conversion kit, I didn't like the proportioning valve that came with the kit (one with a knob). I had allready removed the disc brake system from a 73 chevelle, only to find out it wouldn't work. I ended up using the proportioning valve from the 73, and my master cylinder is the larger one from a 67, and my brakes are better then most later vehicles that I have driven. I had to make my own brake lines from the master cylinder to the proportioning valve, but it was no problem, just have to make sure that you use a double flare on each end.

Front brakes go to the BIGGER reservoir, which may or may not be the rear one.

Front or rear reservoir makes no difference in applying the front brakes first. In fact, there's a hold-off valve to PREVENT the front brakes from applying before the rear drums have enough hydraulic pressure to overcome the return spring tension.


Are those 2000-model year calipers a "low-drag" design? Takes a ****load of fluid to move the pistons out to contact the rotor with low-drag calipers. Thus the stepped-bore "quick take-up" master cylinders.

Factory 2000 firebird/camaro front brakes are commonly referred to as the "LS1" brakes (98+ F-body). Fronts would be twin 44mm piston calipers on 12"x1.25" rotors. Factory master cyl was NOT a quick-takeup deal, so I don't think these are the low-drag calipers (?)

In reality, the twin 44mm pistons have slightly less area than the stock 2+15/16" single-piston calipers, so they should need more pressure and less volume to work effectively. Most folks run a 1" bore master cylinder with power brakes and this setup (and that's the size the factory camaro/firebird used for them too). A replacement master cyl for a 68-76 corvette with MANUAL brakes has a 1" bore and should bolt right up to a factory booster. (It won't have a residual pressure valve for the rear drums, which some people believe may cause a slightly longer pedal stroke)

That said, you should still be able to tune out the rear lockup with the adjustable proportioning valve. The prop valve works by basically regulating the pressure "out" of the valve compared to the pressure "in" to the valve. With a factory brake system, the pressure to the front calipers is the same as the pressure into the prop valve, so it really "knows" front brake pressure even though it's not plumbed into the front system. I'm assuming that the "adjustable pressure valve" you mention is something like THIS (http://store.summitracing.com/partdetail.asp?autofilter=1&part=SSB-A0707&N=700+-48578+115&autoview=sku)

If so, turn the knob completely anti-clockwise and see if the rear lockup goes away. If that doesn't work, try completely clockwise and see what happens (I can't remember which way does what). You should at least be able to tell a difference.

If that doesn't work then:
-- you may still have air in the front brake lines, try to bleed again
-- front/rear brake balance may be too much for just a prop valve. Not sure what the "stock" wheel cylinder and drum sizes are for a 9", but you may be able to move to smaller rear wheel cylinders or drums (?)

Should be able to sort this out without having to go to rear discs, just need to sort out what's going on...

Factory 2000 firebird/camaro front brakes are commonly referred to as the "LS1" brakes (98+ F-body). Fronts would be twin 44mm piston calipers on 12"x1.25" rotors. Factory master cyl was NOT a quick-takeup deal, so I don't think these are the low-drag calipers (?)

In reality, the twin 44mm pistons have slightly less area than the stock 2+15/16" single-piston calipers, so they should need more pressure and less volume to work effectively. Most folks run a 1" bore master cylinder with power brakes and this setup (and that's the size the factory camaro/firebird used for them too). A replacement master cyl for a 68-76 corvette with MANUAL brakes has a 1" bore and should bolt right up to a factory booster. (It won't have a residual pressure valve for the rear drums, which some people believe may cause a slightly longer pedal stroke)

That said, you should still be able to tune out the rear lockup with the adjustable proportioning valve. The prop valve works by basically regulating the pressure "out" of the valve compared to the pressure "in" to the valve. With a factory brake system, the pressure to the front calipers is the same as the pressure into the prop valve, so it really "knows" front brake pressure even though it's not plumbed into the front system. I'm assuming that the "adjustable pressure valve" you mention is something like THIS (http://store.summitracing.com/partdetail.asp?autofilter=1&part=SSB-A0707&N=700+-48578+115&autoview=sku)

If so, turn the knob completely anti-clockwise and see if the rear lockup goes away. If that doesn't work, try completely clockwise and see what happens (I can't remember which way does what). You should at least be able to tell a difference.

If that doesn't work then:
-- you may still have air in the front brake lines, try to bleed again
-- front/rear brake balance may be too much for just a prop valve. Not sure what the "stock" wheel cylinder and drum sizes are for a 9", but you may be able to move to smaller rear wheel cylinders or drums (?)

Should be able to sort this out without having to go to rear discs, just need to sort out what's going on...
Yes, I am using an adjustible valve just like the one you showed there. Accordong to the instructions for it, the more you turn the valve clockwise, the less the pressure should be going to the rear. But like I mentioned before, how does this thing know what the pressure is for the front brakes in order to keep the rear pressure at the correct level? It doesn't, does it?

Also, the master cylinder has equal sized resevoirs front and back. I used a Napa unit ,but I'll need to go find my notes to grt the part number for it, but as I recall, it was a disc brake master for a car or truck from the seventies, that's why I used it. I had already had it for years for another project and had never gotten around to using it.

how does this thing know what the pressure is for the front brakes in order to keep the rear pressure at the correct level?
Front and rear pressure is the same until the proportioning valve gets fed enough pressure to activate. There's no need for it to "know" what the front pressure is--because the pressure for the rear brakes at the inlet of the proportioning valve is identical.

On "normal" stops, it doesn't do a thing.

I've found the part number for the master cylinder I used on my conversion. It's a Napa United part number 36280, which fits several GM models with disc brakes in the late '60's, and it does use the front reservoir for the front brakes. As I recall, I had originally purchased this thing for a '68 Caprice I had years ago that I was going to convert to disc brakes, but never got around to doing. I had since sold the car, but I kept the master cylinder, since it does fit a lot of different models.

Anyway, I also have the original residual pressure valve for the '68 Caprice/Impala. I had another '68 Impala with disc brakes as a parts car, so I would have everything I would need for the conversion.

Now the '68 Impala had the old 4 piston calipers like the ones used on the Corvettes, but just the normal rear drum brakes. Wouldn't the volume requirements for the 4 piston calipers be approximately the same as the '00 Firebird dual piston calipers?

The only thing that I can think of that is throwing a monkey wrench into the equation is the Ford 9" diff with drum brakes. I'm not certain of the wheel cylinder diameter on those, and currently can't check it, as the car is in another garage at the moment.

Also, does anyone happen to know what the wheel cylinder size is for the rear brakes on a '68 Caprice? I don't have access to a catalog that lists that. My reasoning here is that the size of those cylinders compaired to the size of the 4 piston calipers is related to the size of the dual piston calipers and Ford wheel cylinders. Does this make sense to anyone? I think what I need to find now is a master cylinder that would fit a system that roughly matches what I have now, so that the volume of fluid moved to both th front and rear systems is correctly proportionate, allowing the front brakes to do most of the braking as designed.

I know I'm asking a lot here, but does anyone have any information that could help me here?

What I'm thinking now is that perhaps a master cylinder for a truck would have the correct volume transfer for the size of cylinders and calipers I have now. Does anyone have any thoughts on this? Any help you can provide would be greatly appreciated.