Front-rear line pressure bias

[woody80z28]

Converted my Beretta to rear disc and the stock master gave me 1400psi front, 500psi rear. Swapped in a Lumina rear disc master and it brought the rear pressure up to 950. 1400/950 still seems like a pretty big split, considering the caliper pistons in rear are also smaller. If my math is right, the 2.241 front piston creates 5516psi at 1400 and the rear 1.335 piston creates 1320psi. That's a pretty big split and there must be proportioning built in, because there are 4 individual lines coming out of the master that go directly to each corner. The Lumina master I thought would be a good match because the front/rear caliper area is 4.28/1.486 compared to my 3.94/1.39 setup.

Has anyone with an adjustable proportioning valve checked your front/rear lines pressure after you have dialed it in? I'm just curious to see what your split ends up being. And if a traditional disc/disc master has the same pressure out of both ports without a prop valve? It seems to me that I should have more pressure in rear due to the piston being so much smaller as well.

[Hydratech®]

Careful bud, you have no weight in the rear end of that car. *IF* you have functional ABS, you can rest comfortably that it will save your butt in 'interesting' situations. Your numbers DO sound about right from what I have seen in my own experiences, BUT real time wet road testing will tell you if it is also now time to upgrade the front brakes to restore proper front to rear brake balance. Rear disc do need more pressure than the (self energizing) (and lockup prone) drum brakes did in those applications...

To overstate the obvious? Too much rear brake will bring the rear end of the vehicle right around during abrubt panic stop stop maneuvers.

Here is more reading on this: http://www.hydratechbraking.com/braketech2.html

:drive2:

[woody80z28]

Thanks for the reply, Paul.

Another friend of mine pioneered the rear disc swap that I did in his Beretta and went through 3 sets of front pads and the rears still looked new. This was with the stock Beretta disc/drum 1400/500psi master. That sounds to me like the 500psi was not enough pressure for the rear calipers to function properly. And swapping to the Lumina disc/disc master (also a front-heavy FWD car) did almost double the pressure out back. And the piston area front/rear of the Lumina is pretty comparable to my front/rear ratio. I will look up the rotor diameter too to get an idea on that as well as a comparison.

I will be sure to test the wet lockup in a safe environment. My 91 was 1 year before ABS came on Berettas.

Do the typical OEM tandem masters like my Camaro generally have a bias built in? I am running a 70 Vette 1" MC and how only checked front line pressure. Now I'm curious about the back.