I'm using the master cylinder & power booster as well as pedals cage from a 1985 Mustang GT for the brake system in my 1946 Ford pickup I'm also using a proportioning valve from a 73 to 76 F100/150. Everything else is new, lines, hoses, wheel cylinders, master cylinder, booster & all. My problem is this, I cannot get a good pedal no matter how many times I bleed the system. As a starting point in diagnosing the problem I removed the outlet lines on the M/C & blocked them both off, then I tried to pump the brakes but they are rock solid. I then hooked up just the front (which are disc) & got the same pedal feeling as before, nothing. I haven't worked much with proportioning valves before but could that be where the problem is or do I have another problem. I have disc on front with drum on the rear. I can bleed the system til I'm blue in the face but never get any more bubbles out of it, just clear brake fluid. If you have any ideas or experience in this area I would sure appreciate your help.
Thanks

chuck

 

Chuck,

You have to proportion the master cylinder to the size of disc brake cylinders and wheel cylinders that you have used. For example, if you have a 1" bore master cylinder from a disc brake car, yes, it could give you great brakes on the disc portion of your truck. Now add, a couple of large chamber wheel cylinders, push down the pedal and they don't have enough juice to fill the wheel cylinder chambers. You may need to go to a 1 1/8 or larger. Verify what master cylinder size and the wheel cylinders that you have, come from and what the bore size is.

Also, make sure that your push rod connected to the pedal is traveling through the full distance of the piston contained within the master cylinder.

Go to this link: Converting Brakes To Disc From Drum Troubleshoot Problem Areas

and read the technical and troubleshooting section. Like this:


Troubleshooting

Remember this: Whenever you have a brake problem, it's usually something simple. Divide the system into sections and try to determine which section has the problem. A logical, systematic approach will make finding the problem easier! Click here to download a printable troubleshooting flow chart to help guide you through a logical brake troubleshooting process (PDF).
Brake Troubleshooting Guide

If you can’t get a pedal please perform these tests before calling! Click here to download a printable PDF version of these troubleshooting tests. If you are unable to diagnose your problem and decide to give us a call, please have your customer order number ready.

The Most Common Reasons for a Poor Brake Pedal:

The bleeder screws on the calipers are not facing up.
The master cylinder was not bench bled or was not bled completely.
Defective rebuilt master cylinder with pitted cylinder bore or defective pressure seals.
Master cylinder bore size too small for the system volume requirements.
Use of a disc/drum master for a 4 wheel disc system.
Master cylinder lower than the calipers or wheel cylinders.
Lines or components near a heat source.
Lines that loop up higher than the master cylinder and then come back down. Will trap air.
Low drag metric calipers without the use of a quick take up master cylinder.
No residual valve to rear drum brakes.
Drum brake wheel cylinders too large.
Silicone brake fluid ( it can tend to trap air and cause seals to swell).
Rear caliper parking brake / pistons not set properly with a rear disc system.
Rear calipers not being bled properly. Most brake problems with four wheel disc cars comes from the rear.
Improper pedal adjustment with too much free play.
Old or inferior quality brake hoses.

Hope that this helps you out.

 

Thanks for the info. I did spend some time on the phone last week & found a guy in TN that used to race & build stuff, he was recommended by a friend in the Badgerland F110 club. We chatted, he asked questions & I gave answers. The last question he asked was where was the bleeder screw clocked (12 or 1) & when I said about (7 or 8) he said, switch sides with the calibers. With the bleeder screw facing down there was a large pocket of air that was trapped & would never leave. I switched & as I bled them I could feel the pedal getting better & better. The only problem is that even tho with the pedal pushed down I couldn't rotate any of the tires the pedal still went to the floor. As per your info & also 1 of the phone calls I made last week, I now know that the Mustang master cylinder bore (7/8") is to small for the 1978 to 1984 F&G body GM caliper, but the 1984 F250 (1 1/16") should be just fine so that's my next move.
Yesterday was the 1st day that truck was on the road since my dad parked it in 1989 & fortunately the car show was only 3 blocks from home & the E-brake helped when needed.
Thanks
for your help.

chuck