PDA

View Full Version : brake line pressures



1red57
09-21-2013, 11:11 AM
does anyone know what psi readings should be at front power disc calipers and rear drum brake cylinders. 57 chevy. thanks any info will be appreciated

chevynut
09-21-2013, 01:46 PM
It depends on the force applied to the pedal, the pedal ratio, the booster multiplication factor, and the size of your master cylinder bore. The stock pedal ratio is about 6 to 1.

chevynut
09-21-2013, 02:14 PM
What problem are you trying to solve?

1red57
09-21-2013, 05:13 PM
car not stopping like it should. plenty of vacuum 20+inches.pedal really not like I think power brakes should be. brakes bled several times pump up method and gravity bleed. confident no air in system. with brake pressure gauge I read 450 psi at rear wheels on braking and 500 psi if I really stand on pedal. 900psi on front calipers. cannot lock up wheels from 35 mph. everything from front to rear brakes has been replaced and I mean everything. this is a kit which included all new parts. kit came with adjustable prop valve. with valve open drum brakes on rear should lock and then adjust valve to decrease pressure to equalize braking but when adjusting to decrease pressure braking is worse which is understandable an pressure drops to 250 psi midway between open and close so I pretty sure valve is working. I have been in touch with supplier but not much help. easy install with also ssbc pump. thanks for input. grasping for STRAWS.

Rick_L
09-21-2013, 05:41 PM
Most brake setups require 1000-1500 psi to get good performance. This will vary however. Some may need 2000 psi.

Several things affect line pressure. Master cylinder bore size and pedal ratio are the usual ways of changing it if needed.

You might tell us what you have for master cylinder, booster, and calipers. The system must have all the components match.

You mention SSBC. SSBC calipers have pretty small bores, and this means you need higher pressure than some others.

chevynut
09-22-2013, 08:50 AM
900 PSI in front does sound low if you're really standing on the pedal. Just to give you an idea, a 1990 Corvette is supposed to have 1250 PSI in front and 750 PSI in the rear with 100 pounds of pedal force.

Here's a link to the stock 57 brake specs:

http://chevy.oldcarmanualproject.com/chevyresto/57038.htm

The overall manual brake ratio is supposed to be 29.1:1. That means if you apply 100 pounds of brake force, you should theoretically get 2910 PSI....at 50 pounds it should be 1455 PSI.

What size vacuum booster do you have? What is the master cylinder bore size? Do you have a link to the kit you used?

Rick_L
09-22-2013, 06:19 PM
I thought I replied yesterday but don't see it.

I agree with Cnut, but as a practical matter, putting 100 pounds on the brake pedal is probably a bit beyond reality. At least in normal driving. Maybe you get close sitting in the shop, or maybe a true panic situation.