After brake, suspension, and bearing work at all four corners I am now ready to install the new MC, combo valve, and booster for the 4 wheel disc brake system (did a rear conversion) on a '74 scout II. I have the correct components and inquired about the procedure for proportioning/combo valve installation as this is my first brake work other than changing pads. I was told to be sure the front MC reservoir is plumbed to the rear brakes, and back reservoir to front brakes. What is the reasoning for this? I thought the combo valve took care of all the pressure distribution issues, and as I recall the two MC reservoirs are the same size (although I forgot to confirm this prior to leaving for work this am). Is there a difference in function between the two reservoirs?
4W disc master cylinder plumbing
- Thread starter meisterj
- Start date
FDChappie
Well-known member
I'm sure someone here will whack me in the knuckles with an iron rod if this is wrong 
. I think the reason for plumbing this way is that as you step on the pedal, the back chamber is pressurized first. Since most of the braking force, 70%, is achieved by the front brakes, it is a lot less likely that the front brakes will lock up before the rear.
Even plumbed this way on my '74 Scout with OEM disk brakes, the rear drums will lock first when the brakes are applied quickly.
. I think the reason for plumbing this way is that as you step on the pedal, the back chamber is pressurized first. Since most of the braking force, 70%, is achieved by the front brakes, it is a lot less likely that the front brakes will lock up before the rear.Even plumbed this way on my '74 Scout with OEM disk brakes, the rear drums will lock first when the brakes are applied quickly.
Chappie is correct. The rear piston of the MC is the primary and the 1st to pressurize. Once it's past its compensating port, the pressure also acts on the front secondary piston to move it to pressurize the rear brakes. There is a rod that limits travel or can connect the pistons during a system fail, but it's pressure that does all the work in a functional maintained system