Poor braking

[Ackattack]

I'm just about finished with my truck build, and have about 700 miles on it now. My brakes seem to provide inadequate braking power compared to what they should be. I can stop pretty quickly, but I can not lock up any tires (no, I don't have ABS).

This is on a 68 C-10 pickup setup similar to a 72 that had factory front disks.

On the front I'm running some aftermarket dual piston calipers w/ performance brake pads and stock diameter drilled/slotted rotors.
[Dual 47mm pistons]

http://www.performanceonline.com/LARGE-GM-D52-ALUMINUM-DISC-BRAKE-CALIPERS-DUAL-PISTON-PAIR-POL9847-POL9847628/

On the back I'm running LT1 camaro single piston PBR calipers with 94-96 Imapala SS drilled/slotted rotors.

My master cylinder is a new stock disc/drum unit for a 72 C-10.

I also have a disc/disc proportioning valve:
http://www.classicperform.com/Store/1947_59_Trucks/PV4.htm

Additionally I have a line lock solenoid, all of the brake lines and hoses are new. With the line lock set, I can easily roll the truck forward too.

I'm thinking it has something to do with fluid flow and MC bore size. I remember reading some discussions on here about this, but didn't see it today.

Looking for some recommendations on what to check for, cause I know I should be getting better braking performance out of this set up.

[NOT A TA]

Do the brakes work better if you make a couple stops to warm them up?
Were the pads bedded in?
Were the rotors seasoned?
Are the brakes vacume assisted?
If assisted how much vacume do you have?

A lot of upgrade pads need some heat in them to work well. I found with heavy duty semi metallic and ceramic pads I tried that if they were cold the line lock wouldn't hold the car in the burnout box at the strip. So if the staging line was short I'd drag the brakes from my pit to the staging area and if staging lines were long I'd drag the brakes every time I moved up.

[prostreet69camaro]

Make sure you have adaquate vacuum, if booster make sure it isnt bad. Do a vacuum check on it.

I would say it is the bore size of the master cylinder.

You need to know what size bore goes with the brakes you have.

Also go to the part store and ask for a rebuild kit or see if they can tell you what size piston is in the master cylinder you have.

Smaller bore equals more pressure / large bore is less pressure.

I had a similiar problem and went from a 1" bore to a 15/16" bore which is a 300 PSI difference.

[Ackattack]

My engine is a stock internal 5.3L, so it should have good vacuum.

The MC has a 1-1/8" bore. So after doing some reading, I'm pretty confident that is the main issue.

The rear pads are hawk HPS, the fronts are the ones that come s with the calipers (the hawk HPS pads I had for the front were too thick to fit).

The rotors were new out of the box, cleaned up and installed.

Braking really doesn't even seem to help by getting them hot.

Also, when I first got everything running, I put the rearend on jackstands and at idle, could only just barely get the tires to stop turning. I made a few adjustments.....but still.

So if I need a smaller bore MC, any recommendations on bore size and a donor vehicle for the MC that will bolt to my booster.

[NOT A TA]

You can test the vacum assist by steping on the brake then starting the engine, the pedal should drop a little.

[Vegas69]

Check your pedal ratio. I can lock my brakes up at will with a 1" bore master with MANUAL brakes.

[Ackattack]

OK, I've convinced myself its the MC. So do most GM MC's have the same bolting pattern that will bolt up to my booster?

I think I'll try a 1" bore MC.

[TnBlkC230WZ]

Before investing in an MC, I'd look at a set of pads that work at cold temps. Hawk HPS are not a cold temp pad and will have performance issues until they get warm. If you don't know what is on the front, they are probably junk. Look at a set of EBC Yellow stuff or other EU "R90" approved pad. Wilwood's BP10 pads seem to work pretty good cold too with very low dust, but they are not "R90" approved.

[Apogee]

I would suggest swapping to a 1" bore MC. You have 5.37 square inches of front caliper area and about 2.47 square inches in the rear. You could play with pad compounds all day, but odds are that you will gain very little that way. Hawk HPS pads are not high-temp pads by any means and should work fine in street applicaitons since that's what they're designed for. As for the front pads, per the link you supplied they include some semi-metallic pads that should be fine as well at low temps. A little heat typically helps semi-mets, but if you're noticing little to no difference, then it's probably just a low caliper pressure issue caused by having too large of a master cylinder and/or not enough boost. If your pedal is high and firm, then volume (flow) has nothing to do with what you're describing.

Tobin
KORE3

[Apogee]

...I had a similiar problem and went from a 1" bore to a 15/16" bore which is a 300 PSI difference.

Absolute pressures like that don't usually mean much unless you provide an input force, pedal ratio and boost multiplier to go with it. Just to make sure nobody is confused, most people won't see a 300 psi increase by swapping from a 1" bore to a 15/16" bore MC unless they're running system pressures above 2000 psi, basically twice what would be considered normal for a disc/disc sytem.

The 15/16" bore provides about 12% more pressure for a given input force than a 1" bore unit and has 12% more pedal travel to go with it. A 1" bore unit provides 26% more pressure than a 1-1/8" bore unit and similarly, 26% more pedal travel. Percentages are based on the 1" bore as the baseline if you were wondering.

Tobin
KORE3