Hi, I did a master cylinder upgrade to a dual master, but I feel like I don't have enough pressure to stop the car quickly if I had to. The brakes were bled, and it seems to have a high enough pedal, but when I try to apply brakes I feel like there isn't enough stopping power. I also upgraded to a 8 3/4" rear end with 10" brakes recently. could it be that the piston is not large enough for the system? I'm thinking of doing a disc brake swap to help, but if there is not enough pressure for drums, will there be enough for discs?
Hang on brother, before you start swapping out parts , you gotta figure out what's going on.
So first off, the simple stuff;
I gotta tell ya; if you have installed NEW shoes into old drums you have at least One and maybe two problems.
1) the radius of the New shoes will NOT match the radius of the old drums. The shoes will only touch the drum in ONE small window . Having an all-drum car, you now have and I'm guessing, about 10% of it's after break-in potential. It's gonna take a lotta lotta pedal pressure to slow the car down, until the shoes fit again.
2) if you did not have the old drums machined this introduces a second problem; the drum may not be round, and it may be bell-mouthed; either of which is gonna further decrease your brake performance.
The prudent thing to do, if you have done this, is to take your shoes and drums to a competent brake shop and have them matched up.
next, the more complicated stuff;
So lets have a look at how a M/C is supposed to work.
Check out this image; Notice that there are FOUR ports , two in each reservoir. Two are compensating Ports that compensate for pad/shoe wear by allowing more fluid into the pressure chambers. Notice that in this drawing, there are two other ports that allow fluid into the chamber
between the pressure chambers. This chamber, in normal operation, transmits the pedal pressure from the rearmost chamber to the front. To do this, it has to be full of fluid of course. If it is not full, then the back-up mechanical system takes over. Notice the long rod between the pistons, that rod is the back-up.
Study the drawing . Notice that the rear piston is allowed to just slide forward until the front piston is hydraulically coupled to the rear one. At that point the rear piston stops just sliding and commences to push fluid down the line.
Now think about this.
The rear piston does no work until the front one stops.
What happens if the front one does not stop but continues until it can go no further? Well right off you will get excessive pedal travel. Finally when the piston stops, the rearmost chamber will again start pushing fluid.
Now there are only a few reasons that this can happen;
1) there is a failure in the hydraulics
2) the brakes in that system are seriously out of adjustment
3) there is air in the line.
4) the inter-piston chamber has air in it.
In your case; I would "bench-bleed the M/C before you go any further. This is SOMETIMES a lil tricky and it is best to do it truly on the bench, in a vise, to avoid messing up your paint. The trick is to get all the air out of the inter-piston chamber.
If after doing that, that you still cannot get a hard pedal then; I gotta assume there is still air in the line.
this is what I would do with an all-drum car. I would isolate the hydraulic system from the mechanical system.
I would remove all the drums and all the brake hardware down to just the wheel cylinders; leave them alone..... for now. Remove the dust seals and all the shoe actuators but leave the little metal pistons in the wc's. Now you are ready to begin the diagnoses; well almost. For this test those little pistons have to be immobilized inside the wc's. I do this by clamping them in there with substantial C-clamps, all of them. In this configuration, you have eliminated ALL of the mechanical system, except the pushrod and the booster if you have one.
OK so now, if you have a properly working HYDRAULIC system, you SHOULD have a high and hard pedal.
Pump the pedal about three times, and hold the pedal down on the fourth, for say 10 to 15 seconds with a moderate but fixed amount of leg power. The pedal MUST NOT fall under your foot, of it's own accord.. If it does fall, this proves that either you have an external fluid leak, OR the M/C is defective; as the fluid can only be returning to the reservoirs past the piston seals, which is bad.
If the pedal was low to begin with but climbed higher with each successive pump, this proves that the Compensating port is working, which indirectly proves that your Pushrod length is at least NOT TOO long. Now remove your foot and wait, the pedal should return to the top of it's own accord, hitting the bumper under the dash with a nice thunk. When it does wait about 2 seconds then press down again, but with modest pressure, and note how far it goes and how hard it is. It should move down only a little, say around one inch or a tad more, and and then become hard.
Only after you get a high and rock-hard pedal can you re-install all the gear.
BTW,
any Mopar dual-M/C from the 60s to the 80s, with a bore of 7/8 or bigger, will, or can be made to, operate any Mopar A/F-body brakes from 67 to 80, and many others I'm sure... Don't even go down that road. For your problem, this is a non-issue.
refresh your screen, and done
