Hydroboost

[firebob]

I recently got my hydro boost finished up in my 69 firebird. I'm running 2000 Trans am 12" brakes in the front and some SSBC disc in the back. No proportioning valve.
When I hit the brakes hard the front lock up but the backs don't. I know the fronts are supposed to first to keep the rear from swinging around but I'm not sure if the back are engaging enough. I was told that a disc/disc master cylinder had a 60/40 bias built into it. Is that true? I've never heard this before. If it is I may have to swap the lines at the master and give it a try.

Robert

[71RS/SS396]

What master cylinder do you have?

[firebob]

The way I understand it the spring for the front piston in the master is weaker than the back spring so the front one will compress until the back pressure overcomes the rear spring strength and then it will compress as well. I don't think it creates a real bias. Seems more like a timing thing so that the fronts begin to work before the backs do.
I'm running a 1.25" disc/disc master cylinder. Everything I've found and everything I've seen for sale is designed to make the fronts lock up before the rears( so the rear doesn't swing around on you) but this is the situation I already have so I need something outside of the norm. A normal combo valve will make my situation worse by proporting more bias to the fronts. Just the opposite of what I need to do. What I need to do is lessen the front to rear bias. The only way I can see to make that happen is to limit the operation of the front brakes by moving the adj prop valve to the front line.

Robert

https://youtu.be/eaWBd3M9MN4

[Hydratech®]

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

[firebob]

Paul,
I didn't mean to mislead anybody into thinking it had anything to do with the hydro boost setup. That part seems to work great. It's just that this is a brand new install for me from the vacuum assist that was on before. I probably had the very same situation before but just did not realize it. I don't even think it has much to do with the master cylinder. As far as I can tell, it seems to be functioning as it should.
I could go back and rebleed the lines again( I will have to if I break into them to move the prop valve for sure).
I don't really understand the "CC/measured pressure rise output".
Same size tires front/back.
Yes, the back calipers are the integrated E-brake type( I still have to read up the article you linked).
I'm sure the pad materials Front to Back are all over the place. Plus the surface area on the rear pads are probably 2/3 to 1/2 the size of the front pads. Not to mention the fronts are dual piston and the rears are single piston calipers.

I realize there must be quite a mismatch in braking ability between the fronts and the backs. It may be the case that they are so far apart that I can never get them close to locking up at the same time. I would hate to think that I would have to defeat the front braking ability to make them miserable just to get them to somewhat match.

Keep it coming. I'm all ears!

[Hydratech®]

Have you had a chance to read over our brake bleeding 101 article yet? I'm leaning towards the problem being with your rear integral e-brake calipers goofing off (as they are notoriously infamous for). If your vehicle rolled here into our shop, the very first thing I would look at would be these pesky little devils, as they are likely out of adjustment.

Hmmm - it occurs to me that I should mention that most people also get the knob adjustment completely backwards on most typical aftermarket adjustable proportioning valves. Knob screwed all the way in = full pressure to the rear brakes. Knob screwed all the way out = results in typically a 57% reduction in pressure to the rear brakes. Make sure you are not going exactly backwards in your adjustments!

If you find everything is indeed proper, you may want to consider swapping out the rear brake pads to a set with a higher CF rating (meaning a much more aggressive compound). It's going to dust more, have a shorter lifespan, but provides much more bite for the same clamping forces provided. You definitely don't want to detune your front brakes, instead up tuning your rear brakes. Some have gotten so fed up with the D154 style integral e-brake calipers that they have yanked them out and replaced them with the non e-brake calipers (that fit right in place):

Small 2.5" single piston version:

https://www.summitracing.com/parts/wil-120-13900

The low budget alternate would be to source some OE type D154 calipers, such as '86 Monte Carlo (GM G body) *front* calipers.

(this discussion is assuming that your SSBC rear brakes are running the GM D154 style integral e-brake calipers).

The "CC/measured pressure rise output" is (simply?) only speaking of the CC's of brake fluid output as measured with a burette on a test stand - amount of input stroke versus actual CC's of brake fluid output - this is most always linear / constant, not subject to many variables. Measured pressure rise is a problematic variable, as "CC output" has to take up all of the hydraulic slack in the brake circuit first by bringing the brake pads into contact with the rotors before any pressure rise starts to occur. If you have a large amount of slack / compliance in the system (pads not in direct contact with the rotors, small air bubbles still trapped somewhere in the system, brake shoe or caliper adjustments), then it will take a few extra CC's of fluid production out of the MC during brake apply to actually start building pressure to the brakes at the wheels. A perfect scenario would take a much smaller amount of CC's of fluid volume to begin pressure rise (meaning effective clamping force to begin) as there is very little compliance / slack in the system. An improper scenario as described above would take excessive amounts of MC output just to take up the amount of slack in the system before any actual pressure rise starts to occur. The simplest way to look at this is by thinking of a rear drum brake being wildly out of adjustment, in that your pedal will have to stroke quite a way to first take up the slack of the large air gap between the shoes and the drums before actually building any effective pressure / clamping force. Sooooo, if your rear e-brake style calipers are out of adjustment (air gaps between the pads and rotors), you will be getting much more activity out of the front brakes because they are always in a proper state of adjustment (well, unless pad knock back has occurred) when you apply the brakes. Why? Pressure rise is occurring just about immediately in the front brakes because the pads are in contact with the rotors at all times without any slack having to be taken up, while pressure rise to the rear brakes isn't (because you are wasting the CC output from the MC in taking up slack in the system or compressing trapped air bubbles).

Maybe a simpler way to explain is looking at a dual quad throttle linkage? Imagine if one carb was being actuated way more than the other for the same amount of apply at the gas pedal? You would want to do things to make sure that both carbs were being actuated equally, right? The comparison here is that you are pressing the brake pedal, but finding the front brakes are actuating well while the rear are not...

Ok - that gave me a headache, how about you?

Hmmm - where is my beer assistant?