Hydroboost? I understand why you may have titled this thread that way, but it really has nothing to do with the hydroboost unit. Regardless of whether we are speaking of manual, vacuum assisted power, or hydraulically assisted power, this is a discussion of "Brakes 101" - master cylinder and all items further downstream. All input occurs to the master cylinder pistons the same regardless of the method of apply.
(so that certain readers don't start a myth here that hydroboost has anything to do with front to rear brake actuations)
MC operations are subject largely to where the compensation ports are located. The pistons start moving when you apply the brakes, though you don't start building pressure until the comp ports (little drilled holes in the bottoms of the fluid wells) are covered (meaning pistons have stroked to and past the locations of the comp ports). The springs inside of the MC are also variable in their tensions (meaning when they actually start shoving the forward most piston into motion). Also, keep in mind that the front piston will also be shoved forward through hydraulic compression once the rear comp ports have been covered up (rear piston has traveled enough to go past the comp / feed ports). From what I believe I have come to know, the majority of the C3 Vette MC's (and most all aftermarket clones)(which most are C3 Vette MC's in disguise) sit at about a 65 / 35 as measured for actual CC / measured pressure rise output. Wilwood by comparison, went to further lengths to make their 260-8556 MC's (and bore size variants of this same MC design) come in at a 75 / 25 (according to their data)...
Ok, so toss all of that aside for the moment, as we are going to take a look at the primary problem with a 10,000' view, then 5,000', and closer and closer until we weed out your particular problem (front brakes vastly more responsive than rear brakes). The first thing I see is a classic mismatch of braking components (not a matched front to rear brake package). We could likely chase down the front caliper piston surface area, and then also hunt down the rear caliper piston surface area, possibly finding some mismatch. We could also likely find some air still trapped somewhere in the rear brake circuit (which would be the simplest solution to your problem). We may even determine that the front pad set may have a totally different coefficient of friction as compared to the rear pad set (amount of bite per given amount of clamping force applied). Heck, you may even have vastly wider rear tires as compared to the front, meaning the fronts are going to have a lot less traction than the rears. If you have massive rear meats in place, it's going to take quite a bit to get them to break traction. And actually on and on and on... (suspension dynamics too)...
Soooooo, let's go through the diagnostics like weeding a garden - you never know what other weeds may be hiding until you start yanking the obvious, right?
The very first thing I'd like to know is whether or not this SSBC rear brake package is the integral E-brake design type (e-brake built into the rear calipers). If so, I'll put my money on them being the problem (may be wildly out of adjustment). Our brake bleeding article covers this scenario:
http://hydratechbraking.com/braketech1.html Any air still possibly trapped anywhere in the rear brake circuit will kill the rear brake braking clamping force. Any excessive caliper piston travel required to bring the brake pads into contact with the rear discs will also kill the rear brake clamping force. Your exact MC design is also a variable. Speaking of which, do note that you may even have a problem with the MC, in that the cup seal to the rear brake circuit could be degrading (allowing the fronts to build good pressure but lazy to provide good rear pressure).
Check it all out an let us know - we will
