I'm having some serious difficulty getting the drum brakes on the front of my roadster to bleed up. I bench bled the dual cylinder, new GM style MC and can get some brake if I pump the shit out of it. It will not stay up, ( kind of a recurring problem), more than a few seconds. I have checked for leaks and can find absolutly none. The front brakes are '55 Ford pickup, all new except for the backing plates and hubs. The wheel cylinders are not leaking. The push rod on the MC is properly set for full stroke and full return, ( just a hint of slop). I'm totally out of things to try. I'm sure its something dumb. If I open the bleeders to free air while holding the pedal down I get only fluid, no air. HELP!
the front and rear brakes are adjusted up to have some drag on them? so you pump up the brakes and hold them and they stay or the pedal slowly goes to the floor?
As mentioned - adjustment? Also, do you have residual check valves in place? You need 10 pound check valves with drum brakes and modern master cylinders to keep a little prssure on the wheel cylinders.
I have residual valves front and rear because the MC is lower than the wheel cylynders. If I hold the pedal it will stay up. It seems as if the front brakes don't hold, the rear brakes seem to be up and stay up all the time. I took a fair amount of foamy stuff out of the rear probably because of bleeding the front brakes first, (the longer lines of the two). The MC was bench bled and seemed to work well in the vice. I adjusted the shoes out to just touch and then backed off to allow the wheels to run free.
Judging by what you are saying, (no visible leaks, yet it "leaks down") I say your M/C is defective... If everthing you say is right, I say the odds of this being the issue is over 99%.
So what are the rear brakes? You may have a vollum issue that the Master can't balance. GM master - - F-100 front cyl. What year F-100? The Wizzard
Number 3 for adjustment. If they are not adjusted correctly the springs will pull them right back. You need to have a slight drag.
when your brakes are adjusted up, if you give the wheel a good hard spin the wheel should only spin one full turn, i would do this for bleeding anyway, later if you want to slack them off a little and have lots of pedal then fine, i run mine loose enough so i can push my vehicle by myself if need be, i have adjusted brakes up solid when bleeding.
I'm running the MT early Ford style rear drum setup. The MC is a 1" bore, no internal residual valve, not that it would matter as they are not cumulative, with equal volume front and rear bails. The front brakes are the self-energizing Bendix that is very common. The rears are of 1 1/16" bore which should be a good balance with the fronts which are also 1 1/16" bore. If there was a misbalance of f/r brakes they should still function, just not in an appropriate balance, but should still retain pedal. Am I correct? There is no proportioning valve. I shouldn't need one unless the front/rear balance is not workable.
As per 39 all Ford- The master cylinder appears to be OK when I bench bled it. Plenty of fluid and pressure on both outs. I don't have a pressure gauge to check line pressure. I 'll see if anybody I know has one.
I had a problem on my 54 pontiac where I would bleed the brakes and after a couple of days the pedal would go soggy and need to be re bled. After rebuilding everything on the brakes I just layed under the car in desperation and wiggled the brake light switch and it wept some fluid out the terminals. $5 part vs 4 weeks of checking and rechecking and about $300.
The odd one here is everything, except for the front backing plates and hubs, in the whole system is new. Even the brake light switch. The frame is painted and would show a leak very easy. Tomorrow I'm going to replace the MC and give it a shot.
You say it's a GM master. If your bore size is smaller than recomended size for wheel cyl it cannot move enough volumn to load them as needed. Probably why pumping them makes them grab. Tighten up all 4 corners till the wheel just bearly moves and see where your pedal goes. Pressuer and volumn are 2 different systems. Not sure what size boar a F-100 master for the front brakes would be but to much smaller and you will not move enough fluid in one push to apply the proper shoe to drum pressuer. Thew Wizzard
a 1955 Ford F-100 had a master cylinder of 1-1/16" bore. both my `28 Tudor and `36 have F-100/F-1 brakes up front and work great with a 1" bore `68 mustang MC....although i'm sure they both have rear wheel cylinders that are smaller than yours 1-1/16 and are probably slightly less than 1" i'm thinking you have a bad MC
Not necessarly a bad master but more like a inproper size bore master. It is making and holding pressuer. The Wizzard
I feel bore size is probably damn near idea, giving me a slight edge in pedal pressure. The problem is probably a bad master cyl. from the factory. I'll know when I swap it out.
I used a 1" bore on my 55 Lincoln w/stock drums, IMO it is an ideal bore... I still say the master has a defect......... But I will be interested to know.
The Jalopy Journal
