where's the brakes?

[OldRanger68]

I need help. Even though I have been working on vehicles (both professionally and as a hobby) for 55 years. I have encountered a problem I can't solve. I have a 68 F100 that I have been trying to upgrade a bit. Firstly, I put disc brakes from a '72 on it with the power booster, master cylinder and proportioning valve from the donor. The system worked well without problems until the proportioning valve began to leak and was replaced with one from an '85 which seemed to be exactly the same. Then I decided to put a higher geared (2.25) rear end in from an 86 F100. I replaced the entire third member along with the brakes. However, the brake cylinders were frozen up, so I replaced them with new ones. At that point I began having trouble getting the brakes to bleed correctly and determined that the M/C had gone bad. However, I couldn't get a replacement to work either. Parts people wanted to use a computer to find a replacement, which, of course, was impossible. It seemed that the pedal travel was not right, in that I had to push the pedal almost to the floor before the brakes would engage. Upon examination, I discovered that the rod from the booster was engaging the M/C and believed that to be the problem. Then, since I could not seem to find an M/C that was right, I decide to replace everything with a power brake booster/M/C unit from Summit. I struggled with this for a while and when I removed it to attempt to, once again, bench bleed the M/C discovered that there was very little to no pressure at the rear port. I contacted Summit, and they promptly sent me a new M/C. Now, being gun shy I had a neighbor, who is also a pretty good mechanic, come and help me bench bleed and install the new M/C and bleed the wheel cylinders and calipers. However, now with everything seemingly correct, the pedal goes nearly to the floor before the brakes activate. So, now I wonder if I have the lines from the M/C crossed. The new M/C is a corvette style, which is supposedly good for disc/drum combinations. I understand that usually the rear part of the M/C is supposed to go to the disc brakes, but on this one the large reservoir (which would normally) be for disc brakes, is at the front.
Now, before I do anything else, I want to hear from anyone who has used this unit for such a brake conversion about their experiences and get advice on what to do/try next. Switching the lines on the M/C seems to be the only thing left, but I've been so frustrated by this project, I really need help, support and advice before I use explosives. I'm sorry this was so long, but I wanted everyone to know exactly what I have done... so far.

[cep62]

I'd try to make some line plugs for the master cylinder.
To see if it has a pedal without anything else hooked up
if no pedal its a bad m c or linkage issue.
If it has a firm pedal there must be air in the line somewhere.
are the rear shoes adjusted snug to the drums ?

[pcmdub]

Tough to figure out without a few step-by-step diagnosis.
First, is the M/C the correct bore? Too small and it will have high pressure and low volume, and vice versa. So no pedal might be too large? The corvette master usually works just fine but it does come in different configurations. I even have one that is all aluminum (doesn't rust up).

Second, newer M/Cs like the newly built corvette M/C no longer have the drum brake residual pressure valves to hold the drum shoes under pressure. https://www.wilwood.com/MasterCylinders ... =260-13707

Third, if you can pick up the truck off all fours, step on the pedal and see if the front or the rears are engaging. The rears may need adjustment to the "auto adjusters" to get them to catch up. Once the rear's engage, you should get a bit of pedal pressure back. If the front don't engage at all, then you might want to be sure what you are calling the "proportioning" valve is working properly. Most stock distribution blocks are just for the metering light and do not control the disc/drum combo issues. https://www.youtube.com/watch?v=dD1dbNtH-6s

Fourth, you can double check that the rod depth is correct. https://www.summitracing.com/parts/bae-6801279

Finally, just know, that if you don't have leaks, the pressure is going somewhere... So the pedal pressure has got to firm up if the pads are engaging. If the pads/shoes are not engaging, then there is an issue with adjustment or valving.

[Ren]

I'd try to make some line plugs for the master cylinder.
To see if it has a pedal without anything else hooked up
if no pedal its a bad m c or linkage issue.
If it has a firm pedal there must be air in the line somewhere.
are the rear shoes adjusted snug to the drums ?

Exactly that.

Also, pump up the brakes, and listen for air escaping from the cap on the m/c.

Then you will know that the m/c is defective.

[1972hiboy]

I think the responses so far are heading in the right track.
There has been no mention of adjusting the rear brakes, and out of adjusted rear brakes take up alot of brake fluid.
With so many parts coming and going on this project its important to to mention no matter what, bleeder screws are pointed / position at the top of each caliper / wheel cylinder. If the bleeder is below a brake line that's a problem.
I have also had a problem in the past mis-matching brake components with different internal bores and messing with the pedal that way. If the 86' wheel cylinders are significantly bigger internally than the originals, it will take more brake fluid to fill them so pedal travel down further etc etc,
Happy hunting