[lucifershammer]

I'm planning to pick up a kit to measure the brake pressure at the calipers using the bleeder.

What I'm looking for is how much pressure is a good amount of pressure to have at the calipers? I am using stock calipers.

Note that I know different MCs, boosters, etc will give me a different pressure. But, what I'm looking for is how much is good, so that I know when I've got enough (regardless of how I get there.)

Thanks!

 

[sampoerna415]

It should be around 1200 psi at somewhat normal foot pressure, could be more or less it also depends on how hard you can push the brakes. For trouble shooting you would have to compare the pressure at the master cylinder also, if you have more at the master than the calipers might be a bent line or faulty combo valve. This is a bit of a rabbits hole being all the different factors that can come into play.

 

[lucifershammer]

Thanks. I'm looking for ideal pressures.

At this point, everything has been replaced (all new soft and hard lines front and rear, calipers, wheel cylinders, master cylinder, booster, combination valve, etc.)

I'm pretty sure (even though it is new) I have a bad combination valve. Main reason - it's leaking.

But, I also noticed that the front brakes don't bite - if you push on the brakes, it feels like it only slowly starts to apply the fronts, and they slowly grab more and more the longer you hold the pedal. Almost like the metering valve in the front of the combination valve (which has started leaking) is blocking flow to the front and only building pressure slowly, no matter how quickly or slowly you press the pedal. No amount of herculean effort on the brake pedal provides any help either.

First spring project once it warms up is to replace the combination valve with another new one and test the line pressures at the calipers. But, since I'm there, I figured I would test pressures both before and after just to see how it is behaving.

I was just looking for the ballpark figure of what I want to have in order to believe I have good enough pressure to operate the brakes properly.

I'm considering starting by replacing the disc/drum combination valve with a disc/disc combination valve that doesn't have the front metering (but has the brake warning light pressure differential swtich and also has the rear proportioning built in.) That would rule out any metering valve coming into play here (and also remove one possible source of leak.) And I understand why the metering valve is there, and I'm ok with not having it. Plus, I am looking at putting in a new rear end in the near future, most likely with rear disc brakes, so this would be good to do anyway.

 

[Jack Action]

From page 125 of this 1972 Chevelle factory manual, the brake line pressure when there is a 100 lb load on the brake pedal is 805 psi for drum/drum and 1025 psi for disc/drum.

 

[lucifershammer]

Sweet, thanks!

From page 125 of this 1972 Chevelle factory manual, the brake line pressure when there is a 100 lb load on the brake pedal is 805 psi for drum/drum and 1025 psi for disc/drum.

 

[sampoerna415]

Oh FYI I've had some combo valve's act goofy, mostly on the rears, and what you have to do is with the car running try to kick the break pedal through the floor, I thought it was BS advise when I was given it, but apparently it makes the valve cycle and usually fixes things. Maybe air bubbles stuck in the valve, don't know.