[cobalt327]

Hey guys. Got a 59 chevy truck with the classic performance power brake conversion on the frame (not up on the firewall).

My situation is the brakes work great but they always feel too hard, best way to describe it is when I hit the brake pedal the truck slows down but much slower than it should and the pedal is rock hard.

If I do a panic stop I can NEVER lock up the wheels, sometimes under a panic stop the pedal goes down just more than 1/2 way and is rock hard but the truck keeps going.

Is this a problem with the type of pad or a lack of vacuum or pushrod adjustment or what?

15-18" vacuum at idle, autozone semi-metallic pads, new brake fluid. Running the classic performance proportioning valve as well.

Thanks for any advise.

It could be several things. Now, I'm going to assume the caliper/slave cylinder bores match up correctly to the master cylinder bore since you bought this from a reputable vendor. But if all else fails, look back at this.

The pedal ratio is one thing that can effect pedal effort. But usually when going from a manual to a power booster, the pedal ratio will be such that the brakes are too touchy, not too hard. But it's still something to consider, especially if the pedal assembly is different from stock or has been modified.

The most likely cause is a too small booster or not enough vacuum. You said you have 15 to 18 in/Hg vacuum. Depending on the booster size and type (dual or single diaphragm) and the actual vacuum, the range you give goes from borderline not enough (15") to enough (18"). This also depends on what you are using: disc/drum, disc/disc, or drum/drum. If you can give more info on your set up and the diameter of the booster and whether it's a dual diaphragm or not, this could help diagnose it.

 

[cobalt327]

I have tried it with and without vacuum, booster is definitely working. Its a disc disc setup, not sure on the booster. TCI made a power conversion kit with the brackets and new pedal assembly along with the CPP booster and master cylinder mounted on the frame.

How would I tell which booster it is? I think its the CPP 8" booster. Knowing this info where should I go from here?

I can definitely tell that the booster is helping stop the truck but its just too stiff of a pedal and I can NEVER lock up the wheels if it came down to needing to. I pinched the vacuum hose and got 2 stops until the vacuum was gone in the booster and then no more power assist.

Measure the pedal ratio. A diagram showing how to determine the ratio is here.

 

[cobalt327]

Are the rear brakes adjusted?

You said you have the CP prop valve. Is it adjustable? If you added it to the MC that already has it's own prop valve, you may have reduced the pressure to the rears too much unless the MC prop valve was disabled.

No way to change the pedal ratio easily, huh? That's a bummer- that might be all it needs if the rear brake adjustment and prop valve deal doesn't pan out.

 

[cobalt327]

alright guys, ripped out the MC and my caliper shows it as 1 1/64. Would that be 1", 1 1/16 or 1 1/8...?

Might be a honed-out 1". Is it new or reconditioned?

 

[cobalt327]

This sounds like insufficient vacuum. How much vacuum do you have at idle?

 

[cobalt327]

So smooth as glass idle. That's not it, then.

 

[cobalt327]

One line to the rear split at the rear end is fine.

Have you verified there's no rear brake proportioning valve in the MC? Is there an added-on adjustable prop valve?

 

[cobalt327]

chevyrestore- Is there a combo valve on it? I recall somewhere you saying there was. Or an added-on proportioning valve? If you have a prop valve already in the system (in the combo valve not MC- so sorry), and have added another one, you have cut the pressure to the rear brakes too much.

 

[cobalt327]

The following explains what I'm getting at: http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf. If this hasn't been done, the pressure to the rears will be insufficient. A stock-type combo valve is modified in a similar manner.

 

[cobalt327]

I see what your getting at, I was told by CPP if there is a 2 stamp its for a disc drum and if theres a 4 stamp its a disc disc valve and there is a clear 4 stamp so I shouldnt have to modify anything correct?

In a defect-free world, no you shouldn't have to do anything.

The only other thing I had time to do was throw a one person bleeder bottle on the rears, looks like there a good squirt out of there but not a lot of fluid in that squirt.

What would cause that? Its like if someone was holding the pedal and I cracked the bleeder it would squirt out but not as much fluid as what the fronts would have.

This is what prompted me to bring this up. If nothing else cures the problem- I think you should look to be sure.

The last statement above could also be taken to mean there was insufficient volume as opposed to pressure. Like if the MC were a disc/drum unit, having less volume for the rear drum brake cylinders than a disc brake caliper requires.

Who knows- when all's said and done, you may just need a bigger booster.ain: