I rebuilt the brake system on a '79 F250 4X4 frame and I can't remedy the brake petal problem that goes half way down before the brakes start to engage and close to the floor to stop. I've replaced the master cylinder, brake booster, rubber lines, rear shoes-cylinders- springs. I have repeatedly bled the system and adjusted the rear brakes. The only things I haven't messed with are the front brake pads as they still seem to be in good shape, calipers, and the proportioning valve. As I got the '79 as a derelict, I don't know what it's braking was like before. If anyone has some ideas please let me know, thanks!

 

Since you got a new MC, check the pedal pushrod to make sure it doesn't need adjusting. I had the same problem after overhauling my brakes.

You can always put some playdoh down into the cylinder and stick the MC onto the mount to check and see if there is space between the pushrod and the cylinder faces.

 

Soft pedal=air in system. Hope you bench bled the MC before installing to get rid of all the bubbles or used a pressure bleeder. Another thing I do with those big front brake calipers is whang them with a brass or lead hammer when bleeding to make sure no air bubbles are "Stuck" to the inner "Wall" of the caliper. Other than that check the pushrod as bulldogcountry1 mentioned.

 

As mentioned before:

Check pushrod play.

Did you bench bleed the M/C?
If not, you have air in the lines emanating from it. To fix: Have someone stand on the brakes whilst you crack one M/C fitting....slowly. Watch for air/fluid coming out. Do the same with the other one.

 

Thanks for the replies. Don't think its air in the system as I've repeatedly bled them and you can pump a petal up which doesn't happen. I'll check the rod as soon as possible. Thanks again.

 

OK.....mebbe check the vacuum line to the booster and booster check valve..

 

I had both happen to me, found i needed to adjust the rod, then found out my booster check valve was gone on the inside.

 

Is the pedal actually "soft" or does it just travel a long way before getting solid?
In other words, does it feel fairly normal when it does finally engage?

If it's soft and mushy then you still have air in the system somehow.
If it's just traveling a long way then you probably need to adjust the rod between the booster and the master. That's the most common item to need tweaking that I've seen when people renew their systems.
But since you replaced the booster too, it would be a good idea to verify that the pedal-to-booster rod is adjusted properly too. If it's adjustable even?

But the booster-to-master rod is probably the easiest to check. At least it is for me these days because I can lean over a fender way better than I can twist up under a dash anymore!
And you don't even have to remove any fluid lines in most cases. Just remove the two nuts holding the master, pull the master forward enough to work with, and check a couple of ways.

You can take a caliper and check the depth of the piston and compare it to the extension of the rod. Or you can simply turn the rod out a couple of turns and gently push the master back to the booster.
If the rod touched down inside the piston before the flange mates up to the booster surface, the rod is too far out. Turn the rod back in until you can mate up the master and booster just before it bottoms.

I believe the factory spec is for 1/64" clearance between the rod and piston. But don't hold me to that. Someone will know for sure.
This adjustment makes a HUGE difference in pedal feel.

Good luck. Hope it's that simple.

Paul

 

How to check push rod & adjust: http://www.maximummotorsports.com/As.../MMBAK-1r1.pdf

Repeated below in case link ever breaks...

Brake Booster Push Rod Adjustment
1. Remove your stock master cylinder.
2. Measure the pushrod bore depth and record this dimension as D1. Be sure to measure to the very bottom of the radiused pushrod bore (inside the master cylinder).
3. Measure the depth to the mounting pad and record this dimension as D2 (cylindrical piece that protrudes beyond the mounting face of the master cylinder towards the vacuum booster - this is what your measuring [explained in plain English because I can't copy the pics]).
4. Subtract D2 from D1 and record this dimension as D3.
5. Measure the distance that the pushrod protrudes from the master cylinder mating surface on the brake booster. Be sure to measure to the tip of the pushrod. Adjust the length of the pushrod to be slightly less (.005” to .010” than D3 from Step 4. This will prevent accidental pre-load of the master cylinder from the brake push rod.
6. Bench bleed the master cylinder until all of the air is out.
7. Install the new master cylinder onto the brake booster. Torque the nuts to 25 ft-lbs.
8. Fill the master cylinder with fluid and bleed all 4 brakes per the factory service manual.
9. Have a friend firmly depress the brake pedal and inspect all fittings for leaks.
10. Test drive, and enjoy.

I've changed some of the text from the link to be a little more relevant for our trucks...

 

Possible small leak in one of your hard lines?? It happened to me. Found an old mouse nest in the frame and it was holding moisture which corroded the brake line going to the rear axle.

 

You say it was derelict, how long was it sitting ? Do you get good pressure at all bleeders front an back when bleeding ? Might need calipers replaced and make sure previous owner put the old calipers on correct sides ..and on some autos the calipers can swap sides but not bleed out properly ,,,recenter proportional valve needle can help as well if its not bad itself,,and as stated before adjust pedal pushrod which is very possible the issue