41 Mc troubles

[Ooobie]

Ok rebuilt master cylinder on 1941 3/4 ton with kit from dcm,obviously mc needed it after 20 years no activity,wasn’t horrible inside but honed and cleaned and rebuilt,than did both rear cylinders,that said I’m gonna try it added fluid and and starting pumping to bleed,but no matter how many times I pump it never builds pressure,don’t notice any leaks,I can hear internals going back and forth,but no pressure,didn’t bench bleed,but what should I try look at and try I’m very confused usually you get a little pressure but nothing,any help appreciated

 

[Tooljunkie]

There is a few reasons,bench bleeding being first. This being a single line master cylinder,and the amount of air you need to displace for building pressure is considerable.  Reverse bleeding may work, but requires a tool for that. Can’t gravity bleed as its all pretty much flat. 
one pump,wedge pedal,open bleeder. Close bleeder and repeat.  
 

back to rebuilding,the port or ports to fill piston area were checked? 
the hole being so small are prone to plugging. 

shoe adjustment is critical too.the farther the shoes are from drums, theres a chance you run out of pedal. 

[1949 Wraith]

If you don't want to take it out to do a bench bleed, you could try disconnecting the brake line at the MC and pumping out the air till you get fluid.

[JBNeal]

I rigged up a hard line to bleed the master cylinder in the truck...remove the MC top, submerge the hard line in the fluid, pump pedal until the bubbles stop...

[Dodgeb4ya]

We never use to bench bleed master cylinders on brake jobs through the early 80's. Whether pedal or pressure tank bleeding.

Brakes still bled out reasonably quick.

Bench bleeding makes pedal bleeding the brakes much quicker.

[Tooljunkie]

Re-reading op, repeated pumping will not make things happen faster. One pump, crack bleeder. 
rapid pumping of the pedal does not give bore time to refill. Take your time,,let pedal rest at top for a few seconds.

[Ooobie]

Ok after tearing apart again it was pilot error 1 little piece of sandblasting silica from doing the top piece not the body had lodged in the input shaft ,a 1/32 hole caused this ,ty all for help somehow my cleaning was not perfect,it was the little hole to the right of the big one

 

[Tooljunkie]

Fill hole. Go figure. 

[desoto1939]

It is always something small that will mess up your job.  I even had a better situation. Brakes were working for a long time then one day the pedal went to the floor was able to pump the pedal and get pressure. Looked and no signs of brake fluid on the back of the drums.

 

couple days later tested the brakes good brake edal then it wento the the floor, still no lose of fluid.  Pulled the drums nothing leaking. Took the MC out of the 39 Desoto out from under the floor boards. Took it apart did a rebuilt with all new parts and brass piston. Bench bleed it and got a good pedal.

 

I found the problem the little releif insert at the front of the MC the old one had developed a slight tear inthe rubber so this permitted the fluid and the piston to move but not push any fluid.

 

So small part agin was the issue.

Rich HArtung

Desoto1939@aol.com