Brake System Proportioning
For some reason you are not getting enough pressure to the rear discs - here are some questions that come to mind:
1) Are you running a disc / disc spec master cylinder with equal sized fluid reservoirs? If not, I suggest that you install a '77-'79 C3 Vette MC in place of the disc / drum type you currently have installed. You can view this MC if desired in our products section: MC # 8111 with 1 1/8" bore + disc / disc spec (front port = front brake circuit, rear brake = rear brake circuit on this MC)
2) Are you running a disc / disc spec proportioning valve? Disc / drum proportioning valves will cut back pressures available to the rear brakes immensely, as drum brakes are a low pressure somewhat self energizing design that only needs about 400-600 psi to operate, while discs require way more pressure to operate. If you have to make some changes in the proportioning to increase the available pressures to the rear calipers, we suggest you eliminate the factory block style combination / proportioning valve and instead "live line" the brakes and then run an inline type knob or lever style proportioning valve installed in the main line running to the rear brakes. All factory type proportioning valves are designed to always allow full pressure potential to the front brakes, though are preset by the engineers to always pull back the pressures available to the rear brakes to prevent overly aggressive rear braking actions (which can be quite dangerous by putting the vehicle into a tail spin during hard braking maneuvers). Or? Just swap in a disc / disc spec factory style proportioning valve from an aftermarket source. SSBC (Stainless Steel Brake Co) now also has the factory style block type proportioning valves now also available with an external front / rear brake bias adjustment knob too as yet another option.
Ok, so what is it going to take to replumb the brake system if I want to run a knob or lever type of inline valve only? You will remove the factory block style proportioning valve, then replumb the front brake and rear brake circuits to instead connect directly to the ports on the master cylinder (what we consider "live line" in these discussions). Let's assume that we are running a C3 Corvette master cylinder in this discussion for the sake of reference, though do please also note that not all master cylinders are the same (some port locations are reversed between front and rear brake circuits). C3 Vette MC: front port = front brakes, rear port = rear brakes.
FRONT BRAKE CIRCUIT: Connect a 3/16" brake line to the front port of the MC, then route this brake line down to a spot on the frame where you can then install a suitable brass or steel brake line T fitting. Connect up two more 3/16" brake lines to the brass brake line spec T fitting, then route these steel brake lines as needed to the brake flex hoses at the wheels. Your front brakes are now "live lined" right out of the MC output port and now send pressures straight to the front brakes. Summary: Front brake line is simply split to the right and left sides of the vehicle after it comes out of the MC...
REAR BRAKE CIRCUIT: Connect a 1/4" brake line to the rear MC port, then run this line down to where ever you would prefer to install a knob or lever type of inline style "proportioning or pressure control valve" (3/16" steel line is suitable - it will work, though we recommend the 1/4" as the best choice given our experiences). You can essentially install this type of valve anywhere between the master cylinder and the rear axle brake flex hose - some install it so that it is accessible from under the hood, others install it under the vehicle (to where they can pull over, open the driver's door and reach down underneath to adjust), and the hardcore road racers set these up to be adjustable from inside the vehicle (near the driver's seat frame or similar) for "on the fly" adjustment. The clicker types are nice in that they offer a "memory" to the adjustments (example: click 4 worked best on the track, but click setting 3 worked best on the street), while the knob types can allow for slightly finer tuning adjustments (though no way of knowing where you had it set last unless you mark it and count the turns from your reference point during adjustment sessions). Run the rest of the brake line down to the factory style rear axle brake flex hose, where it then splits right / left to the brakes at the wheels. Summary: Rear brake line simply comes out of the rear brake port on the MC, then runs down to an inline knob / lever type of proportioning valve, then continues along to the rear axle brake flex hose and rear brakes as usual.
NOTE: You must now of course painstakingly rebleed your brakes thoroughly of any air, as making front / rear brake bias adjustments while there may still be some air trapped in the system will be ineffective and somewhat pointless (as you'll just have to do it all over again after further brake bleeding).
What you have accomplished here is to allow full pressures as developed by the master cylinder to now be applied to all of the brakes front and rear. You now MUST tune your front to rear braking bias very carefully, as having the rear of the vehicle come around on you during hard braking can result in loss of control of the vehicle (!) (have you ever applied the emergency brakes hard at higher speeds? = same effect!) What you need to do is to carefully run the vehicle over to your favorite large industrial parking lot or stretch of empty road, and start out by making some low speed hard stops. If the vehicle seems fine at 10-15 MPH, now graduate to about 20-25 MPH and see if any overly aggressive rear braking starts to manifest - dial back the pressures to the rear brakes as needed until you feel the brakes are semi balanced. You can also look at the marks you are leaving on the pavement as a visual clue, and having a helper watch from a safe distance is useful too. If you now have the braking dialed in for safe and well balanced stops at these lower speeds, now it's time to make these same test stops at boulevard speeds - get it going about 35-40 MPH and stomp down on the brake pedal and evaluate. Do keep in mind that you are not working towards locking up the brakes, though you are wanting to get pretty darned close to see if there is any overly aggressive rear braking. Also, do not pound your brakes too hard while setting the bias, as if they get too hot you will find them to act completely differently once cooled back down to normal levels - take a break to let them cool down if you feel they have gotten to hot (though don't just park, instead roll slow and easy to keep airflow across the brakes until they cool back down). Continue the adjustment process as spelled out above at higher and higher speeds until you are confident that you have achieved maximum braking bias settings at all speeds. As a general rule of thumb, you should strongly consider setting the rear brake adjustment about 10% less than what your testing is showing you need, as this will help to make sure that the rear of the vehicle will always remain stable under most all braking maneuvers. We also strongly suggest that you retest these adjustments on different types of pavement (concrete vs asphalt is night and day different), and also suggest that you double check your settings on damp pavement too, as you would be quite surprised what a difference in adjustment each type of road surface / condition may require.
WARNING: The information provided here is targetted towards individuals that are very skilled and highly experienced in automotive repair, fabrication and design. If you are uncomfortable with ANY aspect of these procedures, please enlist the services of an appropriately skilled / experienced professional to implement these modifications!
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