The Stovebolt Site Forums The Parking Lot Tons 'o Fun master cylinder bleeding

How can you tell if you have a bad master cylinder? I was tring to bench bleed my new master cylinder that came with my power brake kit for my 59 3800, I put it in a vise and put fluid in both ports and waited for some fluid to dribble out of the line ports, nothing happened, then i took a long philps screwdriver and pushed the plunger into the master and waited for the bubbles to come up, they did but still no fluid came out of the line ports Finally after alot of pushing on the screwdriver some fluid spitted out but not like it should have, i have never had a problem like this with a new master. Am i doing something wrong or is this just a bad master??

Sounds about normal. Push in on the plunger & then put your finger over the hole as you release it. If you take real short strokes with the screwdriver you can watch the bubbles come up.

ya i've done that and still get very little fluid

ok i have bench blead the master put it on the truck and didn't get any fluid to the wheels at all, i blead it again and now i get fluid to the wheel cylinders, i blead them and now i only have front brakes, no back brakes, I have fluid to the back brakes no air comming out when i bleed them but i jacked up the back of the truck and spun the wheels, hit the brakes no stopping, i did the same to the front and they stopped on a dime, what gives? i have found no leaks in the line and when i pushed the brake pedal down and opened up the bleeder screw on the back wheels i have fluid comming out, but the wheel won't stop if you spin it. with no real access to the master except crawling under the truck everytime to fill it and less then happy help (wife) this is starting to become a problem

my first two impressions are this. First when you bled them did you start with the left rear? My second impression is if you have the "plumbing" all done right inside the drum then you need to adjust the rear brake shoes. Sometimes you think they are OK but after spinning them some and applying what brake you have the shoes will "set", or position themselves, and you need to re-adjust them. Adjust them till the wheel locks down and then back off 3 or 4 clicks (I don't remember what the manual recommends)
If you are getting fluid at all wheels and no air then adjustment is about all that it can be.

When i started to bleed the brakes i started with the right rear the farthest from the master then left rear right front then left front i had problems bench bleeding the master, i checked all the connection, at both ends and at the residual pressure valves all's good the fronts stop fine but the backs don't stop at all like they aren't getting any fluid.

You state you have fluid coming out...if so then I suspect the pistons in the wheel cylinder are expanding the shoes but the shoes still need adjusting to be in contact with the drums...also, the left wheel is usually the one farthest away based on the length of the lines. "Most" trucks I've worked on have the brake line goe to the right side then down the frame rail and then across the rear axle making it the Longest(fartherist) from the master cylinder.

Keep us posted on what you find out. I'm curious!!

If you have all the bubbles out at the bleeder on the wheels, then the brakes are fully bled. You may need to adjust the shoes, or check your wheel cylinders. They may not be properly spreading the brake shoes.

For adjustment on the rear brakes, (I'm assuming you have the adjusting bolts with inner gears for HD 1 tons) you are supposed to tighten the bolts until the drum is unable to spin by hand. Then back it off 1/3 turn. This should give the proper clearance and feel for the rear brakes. For the front drums, with the brake adjuster bar (reach-in-style,) you back off 3 to 4 clicks as Achipmunk mentioned.

Good Luck! And definitely keep us posted.