The Stovebolt Site Forums The Shop Area Driveline How to Bleed New Brake System

I finished installing a front disc brake setup onto my 1950 GMC 100 panel. The parts are: front calipers: 1985 C10, rear wheel cylinders: 1966 C10, master cylinder: 1968 Mustang, 1 inch bore bought as a kit with a combination valve mounted under it.

The brakes seem to work fine, but the pedal is unacceptably mushy. I obviously have air somewhere. The pedal is 9 inches from the floor. The first push engages at around 2 inches from the floor and is very mushy. I can pump the pedal multiple times and get the pedal to engage at near 5 inches from the floor, but the mushiness remains.

I have loan of a power bleeder. I made two circles, bleeding at each wheel twice. After finishing the 1st round of bleeding I made a guess that the bench prime of the master cylinder may not have held. I checked that before starting the 2nd round of bleeding. There was no air in the MC, so that guess was not the right one. The 2nd round did not make an improvement over the 1st round.

I have run near a gallon of DOT 3 through it. I do not see any leaks.

I suspect operator error.

Does anyone have advice on what I am doing wrong?

Thanks!

Several years ago, my grandson, who was stationed at camp LeJeune at the time, was helping his friend repair the brakes on his truck, a Dodge Ram, IIRC. They had used up a huge amount of brake fluid bleeding the system, and still had very marginal braking. He called me to ask for some advice. One of the first questions I asked was about the location of the bleeder screws on the front calipers. It turned out they had installed the new calipers upside down- - - - -left caliper on the right side, etc. and the bleeders were on the bottom instead of the top. They swapped them left to right and solved the problem!
Jerry

Are the rear brake shoes adjusted up properly?

Greetings. I had the same problem on my 51 one ton with front disc( 2011 one ton Express van ) and stock rear drums. I utilized a dual master cyl with 7 “ booster. Couldn’t get a good pedal after a few bleeds. Went from a 1 inch bore to a 1 1/8 bore m/c and used a portioning valve rod block when bench bleeding. The insert goes into the portioning valve where the switch is removed. Center the rod, insert the “block” to prevent the rod from moving when bleeding the vehicle. WOW, what a difference, when bled, remove the block and reinsert the switch in the portioning valve.

I also recommend adjusting the drum brakes, make sure the shoes scuff the drums, rotate the drum in both directions and re-adjust so they scuff.

Thank you all for your advice.

The rear shoes are adjusted. I replaced the rear shoes before starting this project. There is no slot for manual adjustment, so I got them close and finished them with backing up and stopping. I just now jacked the rear up and confirmed the shoes do scuff when turning the wheel by hand.

Upside down bleeders? I will not claim to be immune to making such mistakes, but, this time, I do have them pointing up.

I found reference to a pin to hold the combination valve in place during bleeding, but I found that reference after my bleeding, and my combination valve did not come with a pin or with instructions for one. I don't know what I can do with that now. My 1978 C20 has a combination valve. I have twice driven it home after losing the rear fluid through a failed wheel cylinder. The red brake light on the dash stays on for a while, presumably because that rod burying to one side. The light eventually does go out which tells me that the rod will eventually recenter itself. Just a guess. I do not have a light wired for this 1950 GMC, so I will not have that clue.

I like how the brakes are working, except for the pedal mushiness. I can go some distance from my house by 25 mph roads, so I will drive for a while then try a rebleed.

I’ll presume your 50 is an under floor master, do you have residual pressure valves installed?

Chuck

Yes, under the floor. I installed a 2 psi residual pressure valve for the front brakes. I do not know how to check, but my research has me believe that the disc/drum master cylinder would have the 10 psi one for the rear. I believe also that the combination valve also includes the 10 psi residual pressure.

Are my thoughts correct? How can I check?

Thanks!

If it's a under floor master, you could try jacking up the rear wheels so that the rear wheel cylinders are higher than the master cylinder while bleeding.

Disclaimer: Haven't tried this myself but others have reported that this helps.

Just curious….. what does the pedal feel like with the e brake on?

As for adjusting rear brakes ideally you tighten until the wheel locks so you know the shoes are full out, then back off to light drag. Not sure what backing plates and self adjusters are on your rig

Interesting question. I tried it. I gain about an inch of pedal engagement (an inch farther from the floor.)

Your adjustment method is how I do cars that have slots for manual adjustment. I have a 1966 12 bolt differential. It has no slots. I adjust as close as I can and still get the drum on then use the self adjusters for the rest.

The brakes were noticeably much improved on my last drive today, but they are still mushy. I looked further into the mention from pdlightning above about pinning the combination valve. I think mine may have been tripped but reset itself over time. I do not have a pin, but I believe that I will be able to make something work. Tomorrow's task then another round of pressure bleeding.

Something may not be right with how far the piston is traveling inside of the master cylinder. Not knowing what you have as far as your master cylinder, push rod and brake pedal arm are concerned, I can't speculate any further. A piston stroke which is incomplete causes all kinds of problems, including the one you are having.

Thanks!

I ordered the bleeder pin. It is due to arrive on the 8th. I will have that in place before I make my 3rd round of bleeding.

Some points that helps for several bleeding problems:

- Take the front caliper a part with a spacer with thickness of the brake disks and move them during bleeding (had this problem on calipers where the bleeding screw is not exactly on the top of the caliper - maybe because they are from other car / mounting position)

- I use a PIN to fix residual pressure valve

- Take the drum brake shoes completely out (till the wheels could no more spin) and bleed. Than put them back to normal position (this helps to fill the cylinder in retracted position)

Thank you for your advice. I’ve had to set this aside for some time, probably a few weeks. I look forward to being able to get back into it, and I’ll update when I do.