Not enough rear disc brake pressure

[455bird]

Picked up a 1969 Firebird 400 convertible project that appears to have a Blazer S10 rear disc brake conversion, with stock 11" discs (D52 calipers) up front. It has a hydroboost system, with the pedal rod in the upper (manual) brake hole and a 1" aluminum master cylinder from a 1997 Buick fullsize (4 wheel disc car) along with an adjustable proportioning valve on the rear brake line. There is no factory proportioning valve. The problem is that the proportioning valve is backed all the way off and the front brakes still lock way before the rears. In fact the rears never really lock. I don't see any residual valves in the master cylinder or anywhere in the rear brake line. The car stops fine under normal driving, but when driven aggressively, it just doesn't feel like it's getting the most out of the rear discs. Any ideas of what is wrong? Would more aggressive brake pads like Hawk sells help with the rear grip?

[Ron Sutton]

Which direction do you have the proportioning valve knob adjusted ... all the way out or all the way in?

[Apogee]

When you say the prop valve is backed all the way off, you mean dialed all the way in right? All the way in (clockwise) is the minimum pressure reduction...all the way (counter clockwise) out is the maximum. A quick pressure check front and rear would confirm this if you have a brake pressure gauge.

As for your brake bias, it's important to note that your front D52 calipers have ~6.7 square inches of piston area, approximately 30% more piston area than the OE S10 Blazer calipers with twin-46mm pistons (~5.15 square inches), so assuming you don't want to change the components being used, then your best option to increase the rear brake torque may be to run a higher CoF pad in the rear. Alternatively, you could run a lower CoF pad compound up front and achieve essentially the same effect, albeit with more pedal effort required to achieve lockup, although that's not typically an issue with hydroboost, especially installed with a manual pedal ratio and arguably small MC bore size for your given components.

Tobin
KORE3

[455bird]

Yes, the prop valve is turned all the way clockwise or in. Other than changing rear pads, what are my options for the front brake calipers to match up with the rear S10 calipers? Or would I be better off changing the rear calipers? Is a 1" mc too small? What size would you recommend? Does anyone make an aluminum mc with a plastic reservoir similar to what I'm already running? I prefer to stay away from the old style cast iron units.

Thanks for the help!

[Apogee]

You could swap out the front calipers for one of the aftermarket D52 options with a smaller piston area, thereby reducing the front brake bias relative to the rear, giving you a more balanced system.

Not that there aren't other options, however Wilwood has a 2-piston D52 caliper option with 6.28 square inches of piston area or their GM-III calipers with either 4.45 or 5.98 square inches piston area that you might consider. The D52's have bushings and are a bit more streetable, however only offer a ~7% reduction in piston area, whereas the GM-III's would be a more substantial difference.

I don't know of any rear caliper interchange options with larger piston areas, but that's not to say they don't exist.

Most hydroboost installations run a 1.125" bore MC or larger depending on the application, calipers, etc. I don't have a specific suggestion for MC, maybe someone else will, but your first order of business should be to balance your system, since nothing at the firewall will correct for that.

Tobin
KORE3

[455bird]

Is reducing the piston area on the front calipers acceptable for performance driving? Or, is more piston area better? As for the rear calipers, the S10 units have 2.80 sq. in. of area (48mm single piston) and assuming a 65/35 percent front to rear balance, then I would need a rear caliper with approx. 3.6 sq. in of piston area to balance, if I keep the D52's on the front. A mid 90's caprice with rear discs used a PBR single piston 54mm caliper. The rotor width and diameter were the same as the S10. Do you know if the caliper will bolt up to the S10 bracket?

[Apogee]

Is reducing the piston area on the front calipers acceptable for performance driving? Or, is more piston area better? As for the rear calipers, the S10 units have 2.80 sq. in. of area (48mm single piston) and assuming a 65/35 percent front to rear balance, then I would need a rear caliper with approx. 3.6 sq. in of piston area to balance, if I keep the D52's on the front. A mid 90's caprice with rear discs used a PBR single piston 54mm caliper. The rotor width and diameter were the same as the S10. Do you know if the caliper will bolt up to the S10 bracket?

Runing balanced calipers front to rear would be best for performance driving, calipers with smaller piston areas just operate at higher pressures to achieve the same overall effective brake torques. The traction your tires have should be the limiting factor, not your brakes. Bigger piston areas create more clamping force for a given caliper pressure, however require more volume and therefore larger bore MC's...6 of one, half dozen of the other, it's the same thing in the end.

As for your question concerning the S10 and Caprice/Impala SS rear caliper interchangeability, I have no idea.

Tobin
KORE3