Helping a guy with a brake issue-power booster and master underfloor-residual valves front and rear-dual disc drum master. I took the fronts apart and then bled the system- bench bled the master-no air anywhere--fronts lock up rears barely engage lightly. I adjusted shoes out to slight drag and bled rears again, When brake applied lots of pedal but minimal braking on rear-they do engage but not much. There is a proportioning valve as well and it is open--this one has me baffled. My next move is to see if residual to rear is wrong--it has a red to rear and blue to front. There is also a bit of slack in pushrod,
Tim, Is this a new build or old parts ? Could the rear flex hose be bad blocking the fluid flow ? Are the master and wheel cylinder compatible sizes ? I had a mis mach on a project car I bought, the rear wheel cylinders needed to be changed. The brakes worked but not well.
What rear brakes are you working on? Sounds like drums, but self energized or not? Sent from my iPad using The H.A.M.B. mobile app
Slack in the push rod can do it, try to lengthen it but count the distance or # of turns and go by 1/8" to start. Look for slop in the pedal linkage which can absorb motion.
Rear is a Ford 8" Rod seems right--will try it a bit-suspect master all parts in rear are new as is master etc-new lines too-I will play with it some more for him I don't know where he got the master cyl-adjustment between master and booster is right-brakes aren't my favorite!!
Several things need to be verified; Pedal ratio, that is normally in the 3-4:1 range with power assist. Master cylinder can be fully stroked before the pedal bottoms out. Pedal, with it's own return spring, can be moved farther back when disconnected from the booster. This ensures the booster is fully returning. The pedal push rod should be directly connected to the booster. The only free play here is the normal movement in the booster control valve. The booster output rod or pin should have about .030"-.040" clearance with the primary master cylinder piston, and this is checked with full (about 20" Hg) vacuum applied to the booster.
THX V8 Bob-all that looks fine thus far-will ck further for him--already checked clearance inside booster to master and rod free play-may pull master away again and ck clearance from pin to master--I didn't assemble this thing so have to go over it all I guess Lots of pedal now (down about 1/2 way) and fronts locked up tight when applied-rears just a slight drag
I’m not saying this is your issue but just as a FYI. Had a friend a few days ago go through 5 Napa supplied 1940 master cylinders. Had to change to a different manufacturer.
I had a similar problem when I was building the wagon and after several days I finally isolated the problem and it turned out to be the proportion valve, I replaced it and my brakes work like they should. HRP
^^^^^ Oop's one circuit got bleed and pressurized and caused the differential valve to shift on close off other circuit?
OK--new master disc /drum installed-bench bled etc-bled lines-wilwood proportioning valve wide open-blew out all lines-checked residual 10# valve ok-still minimal rear brakes--baffled-have installed many of these setups without issue. Lots of front brakes-vacuum is about 14-15 inches-7" dual diaphram booster under floor-pedal drops a bit when engine fires.
Don't ya wish you had a pressure reading gauge on very gadget and fitting on your brake system? Well that's not possible so what I have been doing is applying pressure to M/C and one by one cracking the line starting with the closest to the M/C and working in the "flow" direction. You should be able to tell when the "bleed" is strong or and where it starts to peter off. It's time consuming, it can be messy but sounds like you have already paid your dues.................................. Do you have the front & rear lines going to the correct port on M/C?
Wheel cylinders with a smaller ID will extend farther for the same pedal travel. This comes up sometimes when the MC and WCs were from different vehicles.
gonna attack it again but then may tell him to take it somewhere else-trying to help but don't know whats next.
failed flex hose connecting the line to the rear axle? this has come up repeatedly on here. an internal failure of the hose blocks off the rear brakes.
Interesting question/problem. I wonder if it isn't the proportioning valve. I am setting up the brakes on the Stude and have all new brake parts. Went to bleed the back brakes (with my homemade pressure bleeder) and get brake fluid to the proportioning valve and it just stops there. I have a call into the vendor for the brake setup (a Studebaker vendor) that included the residual valves and prop valve to see if he has experienced a similar problem. The Prop valve I have is stamped APV1 12M127.
Have some one apply the brakes start at the back and loosen line fittings and work your was forward until you find pressure.
Had a similar problem and it was because the drums were a different size (bigger ) than the shoe diameter......thus only contacting a bit, . can you adjust the rear brakes tight enough to lock up the wheels.
Like this one? It is about as basic as it gets if the knob is turned the right way. https://www.summitracing.com/parts/wil-260-8419
I'll over-adjust the rears at times to bleed and get the best pedal out of it possible. Of course back it off before driving. I also break the leading and trailing ends of the linings with a wood rasp so they break in easier.
drums and shoes arced-all hardware new-rebled a few times and readjusted-have rears now--THX all-did chg master as well. Prop valve is about 1/2 way now-but didn't get much brakes until I opened it wide open and took all the lines out from MC to rear end then to wheels and blew them out into a bottle-didn't see any junk.
I start with the proportioning valve wide open and adjust from there. Glad you got it sorted out. A total shame that parts like a master, there are so many bad ones brand new in the box.
The Jalopy Journal
