Technical Quick brake question

I have a brake problem I am seeking opinions on. The system is very simple; a fruit jar single master cylinder, one circuit to the two rear wheels that have brand new early Ford slave cylinders. Everything is brand new. Charged and bled (several times) with DOT 4 fluid. Just about as simple as you can get. Moy problem is the first press of the brake pedal goes clear to the floor, no pedal at al. It will pump up with a couple strokes to good brakes but always defaults to zero brakes on first push. I've pump bled, vacuum bled, and left pressure on the pedal overnight which sometimes gets rid of gremlin air in a system. There are no leaks anywhere.
Sound like a bad seal in the master cylinder? It is brand new but stranger things have happened. I am down to pulling it but it is a bear to get at so if someone has a magic solution that will fix it w/o removing I would be thrilled.

 

Did you adjust the brakes? If there is too much clearance at the shoes, it will have too much travel and do just what you describe.

 

You didn't say if you had front brakes. If the brake shoe adjustment is ok I would disconnect the line at the master, plug the master and see if it is rock hard or goes to floor. That will tell you if the master is bad. Then plug the front line and see what happens and then the rear line. If it is not brake shoe adjustment or bad master, you still have air trapped somewhere.

 

Is the master on the fire wall or the frame ?? If it below the wheel cylinders you need to install a residual valve

 

Do you have a residual valve either in the master or in line to the rear brakes?

 

I think you guys nailed it. I went to bed last night and as soon as my head hit the pillow it hit me; master cylinder is probably 6" lower than the wheel cylinders. DUH!! I think I have a couple 4psig and 10psig backpressure valves in my stack of stuff. Will install one this weekend and that should solve my problem.

 

Most of those early master cylinders come with a residual pressure valve inside them....

 

Like Jim said, a fruit jar master should have a residual built in. I would give the adjustment a second look; tighten the star wheel as tight as you can get it, then back off to slight drag. I found doing that gives better results than just adjusting to a slight drag; maybe gets everything seated better; who knows; but has cured problems such as your describing more than a few times.

 

These are 1940 Ford non-self energizing drum brakes. Each shoe is adjusted separately and I have them already dragging. Don't think that is my problem. I don't have a soft pedal that a lose adjustment gives I have NO pedal. Will try the check valve, if that doesn't work will bite the bullet and pull the fruit jar.

 

The bleeder screws are at the top of the wheel cylinders, right?

Sounds like a fun challenge...hope you figure it out. I'd go over all the fittings again to make sure they're tight and not leaking, then pull the m/c.

 

Yep. Only compromise is they are front wheel backing plates on the rear wheels so the big and little cylinders are backward but since the car will only be driven (hopefully!) in car show parking lots @ <<1mph, shouldn't need lots of braking power. No leaks, sitting on super clean shop cement floor, no drips for 3 months.

 

How about the travel on the pushrod?

 

This is a good one to check. Once you do get a pedal, see if it holds or sinks to the floor, this is known as byp***ing.

 

I don't read anywhere in your posts if you bled the Master FIRST,then run your lines and bleed at each wheel

 

Kinda off on a side track; but when did the idea you have to bench bleed a single bore master start?

 

Probably with guys who started out working on dual cylinders, and never knew there was a difference

 

It started with me about 60 years ago when I built my first A-bone..

 

Rod is adjusted fine. I am pretty sure check valve will cure the problem. will install it tomorrow and report back.