Not sure what is wrong with the brakes on my 1968 Chevy C10

[HeavyHauler]

The brake pedal on my 1968 Chevy is very,very,very mushy and I can push the pedal all the way to the floor with very little effort and it doesnt stop the truck until you floor it.

It Feels like there is still air in the system although I have bled the master cylinder twice while off the truck and I tried bleeding the system with no success. I then took the truck to a local shop and they couldnt fix the issue either.

Here are the modifications that I have made to the brake system on my 1968 Chevy C10 fleetside short bed. I converted it to 4 wheel disc brakes. All parts are brand new. The truck originally had 4 wheel drums.

*Front disc brake kit from Early Classics
*Tuff Stuff 9 inch booster TFF-2224NB
*Tuff Stuff brake booster rod and clevis kit #4750 (replaced stock rod)
*Master cylinder for 1975 Corvette
*Summit Racing GM prop valve for 4wheel disc system #760185
*Rear disc brake kit from Captain Fab that uses 76-78 Cadillac Eldorado calipers with built in e brake levers and rotors from 92-02 Chevy Astro AWD van

I am getting fluid from all four calipers,bench bled master cylinder twice and bled system numerous times with no success but each time it seemed like the system didnt have any more air in it as I wasnt getting bubbles anymore.

Again also took the truck to a local shop which I dont think the tech was very knowledgeable and he was not able to solve the issue either.

Any suggestions on what might be causing he issue?

[franken]

Just a couple fairly obvious suggestions you probably already eliminated...
Make sure all bleeder screws are facing up. People have managed to install calipers wrong.
Bleed the longest line first and the shortest last.

[lil hoodlum]

I read that it is a powered brakes. Was it originally manual brakes?

[Steeveedee]

Did you adjust the rear calipers? They need the space, if any, taken out. That's different from the front.

[68 P.O.S.]

Do you have the correct size master cylinder? Is the booster pushrod adjusted to the correct length? Is the pushrod in the correct hole in the brake pedal assembly?

Here's a link to CPP's troubleshooting:
classicperform.com/TechBook/BrakeTroubleshoot.htm

[gmc684x4]

Look over all lines looking for leaks

[68Stepbed]

Sounds like a bad master cylinder.

[Boog]

I had the same symptoms. Adjusting the push rod between the master cylinder and booster removed much of the pedal travel from my 69 stock system. I have a good pedal now.

[BigBird05]

2X on adjusting the rear brakes. the combo valve might not be holding any pressure on the rear calipers.

[HeavyHauler]

Originally Posted by franken

Just a couple fairly obvious suggestions you probably already eliminated...
Make sure all bleeder screws are facing up. People have managed to install calipers wrong.
Bleed the longest line first and the shortest last.

Yes all bleeder screws are pointed up and I have tried any and all bleeding methods.

[HeavyHauler]

Originally Posted by lil hoodlum

I read that it is a powered brakes. Was it originally manual brakes?

Yes it was originally 4 wheel drum manual brakes, but ALL of that stuff has been replaced with new updated stuff. None of the original brake stuff is being used.

[HeavyHauler]

Originally Posted by Steeveedee

Did you adjust the rear calipers? They need the space, if any, taken out. That's different from the front.

I do not think that I did that properly so I will have to review and make sure it is.

Would that cause a mushy pedal?

[HeavyHauler]

Originally Posted by 68 P.O.S.

Do you have the correct size master cylinder? Is the booster pushrod adjusted to the correct length? Is the pushrod in the correct hole in the brake pedal assembly?

Here's a link to CPP's troubleshooting:
classicperform.com/TechBook/BrakeTroubleshoot.htm

The master cylinder is for a 1975 Chevy Corvette and the proportioning valve is the same.

I will post a pic of the hole I am using.

I have full pedal travel but it is mushy. Not sure what else to check as far as the booster pushrod beind adjusted to correct length?

[HeavyHauler]

Originally Posted by gmc684x4

Look over all lines looking for leaks

Yep I did that so many times and found some in the beginning but now there are none but will keep checking just in case.

[HeavyHauler]

Originally Posted by 68Stepbed

Sounds like a bad master cylinder.

Master cylinder is brand new. I know even new parts can be bad, but how do I check for it being bad?

[HeavyHauler]

Originally Posted by Boog

I had the same symptoms. Adjusting the push rod between the master cylinder and booster removed much of the pedal travel from my 69 stock system. I have a good pedal now.

I dont remember the booster rod between the master cyldinder and the booster being adjustable at all on this particular Tuff Stuff booster.

[HeavyHauler]

Originally Posted by BigBird05

2X on adjusting the rear brakes. the combo valve might not be holding any pressure on the rear calipers.

Yeah this is something that I will definitely have to check as well as i dont think that I adjusted the rear calipers properly.

[HeavyHauler]

I appreciate all the responses and I can now start checking on these things to make sure everything is in order!