C-10 brake pedal problems

[Mr Twister]

Hi
I have a 1972 C-10 4x4 brake problem I need some help with please. The break pedal falls to the floor about I out of 15 times I use the breaks. If I pump it a few times it comes back up. I installed a new, not rebuilt, master cylinder and bled the lines. Much better pedal now but stills falls to the floor occasionally. I don't see any line leaks. Could the proportioning valve be causing this?
This is not a show truck. I've had it about 35 years and three motors later it's still on the job.
Any advice would be appreciated.
Thanks!

[Mbeef61]

Did you bench bleed the master before install?

[Mr Twister]

Yes, I bench bled the master cylinder prior to installation.
Thanks

[El Dorado Jim]

possibly a bad master cylinder, it is new but could be bad,

[Mr Twister]

Thanks
I've had new bad parts in the past but wasn't Sure about the proportioning valve. No way the proportion valve could cause this?
Thanks for your help.
Mr Twister

[demian5]

Originally Posted by Mr Twister

Thanks
I've had new bad parts in the past but wasn't Sure about the proportioning valve. No way the proportion valve could cause this?
Thanks for your help.
Mr Twister

No, sounds like improper adjustment of the shoes or something not put together right. Pull your drums of any wheels you worked on and look them over good.

[AussieinNC]

Remove the switch from the proportioning valve and re-bleed to brake system, preferably with a pressure bleeder system....start at drivers rear, passenger rear, passenger front then drivers front.

When done, replace the switch...check the end of the switch pin for damage...

I have had numerous trucks come to me that had similar issues and it is usually the prop valve switch locking out the rear circuit then releasing it...

[Mr Twister]

Thanks for the info. I'll take a closer look at the proportioning valve switch. I have a feeling this could be a problem. Do they still make proportioning valve repair kits?
Mr Twister

[Mr Twister]

OK Fellows
Finally have time to work on the brakes again. I installed a new after market proportioning valve for disk/drum brakes. Bled the system twice using a vacuum pump. Still having trouble with the pedal falling to the floor every so often and occasionally the warning light will still comes on. If I lightly tap the pedal it will go out. Could there still be air in the proportioning valve. Before attaching the brake lines to the proportioning valve I gravity bled the valve.

What's happening...how is it possible for the pedal to fall to the floor. Does the proportioning allow the fluid to pass back into the master cylinder.

I'm really stumped here.

Mr Twister

[Ventanakaz]

Curious, Do you have a booster on this truck ?