The Stovebolt Site Forums The Shop Area Engine Shop Bleeding brakes, but no pressure

There's much in the archives on bleeding brakes but nothing that addresses my specific question.

Trying to bleed the brakes on a '51 3100. I've completely overhauled the master cylinder and rear wheels (honed the MC and put a kit in it. New shoes, drums, 1 new WC, 1 WC kit on rear wheels). I have a shop manual and have followed the steps for adjusting the brakes following a rebuild.

When I push on the brake pedal, I can pump it a hundred times and I get no resistance. I have the left rear bleeder screw opened into a hose which goes into a jar with fluid in it, but nothing is going through the hose.

Absolutely no pressure or resistance in the MC makes me wonder if I've over-honed it, but the new rubber cups seem to fit really snugly in the cylinder.

If I take the filler cap off the MC and gently pump the pedal, I can see a little turbulence in the fluid, meaning something is moving down there. The cap has a vent hole in it, so I'm having to hold my finger over it when pumping.

Does anybody have any idea what I can troubleshoot next?

Did you bleed the MC after installing? There might be air trapped within.
Laurence

Hmmm. No, I didn't bleed the MC. The shop manual doesn't describe that step. I suppose I just need to disconnect the brake line from the MC and pump until I get fluid coming out of the MC, then I can eliminate that part of the system as the problem?? Thanks for the tip.

Use the same technique with the MC as you would with the WC; ie: bleeder tube immersed in fluid jar. Let us know if that solves the problem. Another possibility is a reversed MC valve assembly or blocked fluid return/bypass port. And don't block the vent in the MC cap.
Laurence

Cracking the lines at the M/C and letting gravity flow the fluid might do the trick but then, it might not. When you let up the pedal you've got the potential to suck air back into the M/C.

Normally when you bench bleed a M/C you add 2 little plastic fittings with 3/16" hose barbs in place of the normal line fittings. A clear neoprene line runs back into the fluid reservoir and is submerged. This way, when you pump it and release the piston, it won't suck air back in.

You may need to get an assistant to pump a couple times with the fittings tight, hold the pressure, crack the fitting and let the air escape, hold the pedal down until you retighten the fitting. Not going to guarantee this will work....I've had it go both ways. If the lines have been charged or you've been able to gravity bleed the system you've got a 50-50 shot. If not you'll probably need to hustle up a couple extra fittings and make your own return tubes. If it's a stock single outlet you'd only need one. The key is to keep the M/C from drawing air back when the pedal is released. Hope this helps.

Dave

you need to bench charge your mastercylinder.......set it in a vice and fill with brake fluid....then set a can under the front of the mastercylinder........then push on the brake piston lever...fluid should squirt out front of mastercylinder into pan...then before you bring lever back put your finger over front hole so that you dont get any air back in cylinder...then repeat the process until all air is out...you will have to add brake fluid after 1-2 pumps.......then you will know for sure that it is working.......if the master cylinder is working fine then install it.....then if you pump the brakes and get nothing you may have a blocked line

Thank you all so much for the advice. I'll tackle this tonight and let you know how it goes. I had a hunch it wasn't going to be as easy as I was trying to make it.

One other question: I cover the vent hole in the filler cap because fluid squirts out of it when I pump the pedal. Does this indicate anything, perhaps a plugged line or overfilled the reservoir?

I don't think that you should have fluid squirting out the vent. If you pump the MC and the fluid doesn't go down trying to fill the system, you have a problem with the MC. Did you adjust anything on the linkage? The piston has to move back past a certain point in order to get more fluid in the chamber.

dj........when you push the plunger it will compress the brake fluid......so even if you had the front end of the mastercylinder closed it would still squirt out some if you have the top vent cap removed.....it sounds like a gumbed up flex brake line to me.......they will get nasty before actual solid lines get gumbed up.......try the bench test.....then if its working good re-install and then disonnect the flex brake line in front from solid line.......you will find out quick if thats the problme........god..arent brakes fun...wink wink

Thanks guys. Truckernix, I didn't intentionally adjust the linkage. I made note of the location of the jam nut (up against the yoke) before disassembling, so it should be very close to where it was.

I'll try as Butch suggests and see what happens. The truck sat for years with the brake line disconnected fromm the MC, so there's no telling how many bugs etc. may be in the line.

Problem solved! The root cause of this particular problem is that---I'M AN IDIOT! In the end, I disassembled the MC that I had just put a kit into days ago, and I had assembled it such that the primary cup was blocking the "exit hole". In other words, I had the primary cup on the wrong end of the spring. It should've been up against the piston, and I had it down by the valve. That explains why fluid was coming out of the vent hole in the filler cap--it had nowhere else to go!!

Lesson to be learned by all: if you have a shop manual with step-by-step instructions and exploded diagrams, use it! I reassembled this thing from memory because it only has about 6 pieces, but I screwed up.

Once I changed things up, I bench-primed the cylinder as some of you advised. Now, I won't mention that once I started bleeding the line, I had an additional bleeder valve open and pumped fluid on my shop floor. That would be too embarrassing to admit.

Thanks for all the help. Live and learn.

Glad to see you've fixed the situation. The reason I earlier mentioned reversed assembly? Well, now we're a club of two. ;O)
Laurence

Laurence, I re-read your earlier post, and you called it! Thanks for the help.

The important thing is that you figured it out without pulling too much hair out. On to the next project!