My new 78 crew had a newer booster and master cylinder but they were for single piston calipers instead of duals so I swapped them out for the correct ones today. Bench bled the cylinder and vacuum bled the lines. Had lots of pedal and it felt great so I took it for a test drive down the road and back, roughly 1.5 miles round trip. By the time I got it back the brakes were hot and all 4 wheels were tight. Jacked it up and can't roll the tires by hand. Any suggestions? Do proportioning valves go bad? Could I have sucked something into the valve and plugged an oriface with my vacuum pump? Did I just get a bad master cylinder? I did crack the fitting for the front brakes and it freed up the front to where I could roll them by hand. Didnt do the same for the back yet (wife was holler'n to get cleaned up for supper) ill try it in the morning. Am I on the right track with a bad, new master cylinder or what?

The proper calipers are single piston why did you get the dual piston?

I know this may sound crazy..but did you use Dot3 brake fluid? I say this because i have run into an occasion where an individual poured trans fluid in the master cylinder by mistake...your simptoms are very similar to what his were.

BuzzGun79 said:

I know this may sound crazy..but did you use Dot3 brake fluid? I say this because i have run into an occasion where an individual poured trans fluid in the master cylinder by mistake...your simptoms are very similar to what his were.

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I agree ^^ Also the problem could be with the brake line ( rubber hose ) has swollen shut it will allow the pressure to the brakes but wont allow the pressure to release hence the brakes stay locked

The brake hose issue wouldnt likely affect all wheels though would it?

Workin' Rig said:

The brake hose issue wouldnt likely affect all wheels though would it?

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Realisticly...I dont see how Workin...But im getting tired of being misjudged on here... for i cannot always put my thought process into words so to speak...

BuzzGun79 said:

Realisticly...I dont see how Workin...But im getting tired of being misjudged on here... for i cannot always put my thought process into words so to speak...

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I have the same issue sometimes didnt mean to offend you bud.

Sounds like a master cylinder issue to me

Workin' Rig said:

The brake hose issue wouldnt likely affect all wheels though would it?

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The brake hose issue will affect the front brakes more, but you still have one hose that goes to the rear axle.

Brake fluid will go bad, even in the bottle just siting on the shelf.

Something you may want to try is a full flush on the brake system.

Yea but theres one rubber hose to each front wheel and one in the back. So in order for a brake hose to be an issue and lock all the tires all 3 would have to fail at once....kinda unlikly

Workin' Rig said:

Yea but theres one rubber hose to each front wheel and one in the back. So in order for a brake hose to be an issue and lock all the tires all 3 would have to fail at once....kinda unlikly

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For a break hose to fail by swelling shut, its due to old age or contamination so yes any rubber or seals in the system will fail at the same time.
The trucks brake bias is always heavy to the front, so the issue will be more noticeable on the front then the rear.

I had a van that busted a line out of state, so i quick patched it to get home. i had no break fluid so i used hyd jack oil. it caused all rubber parts to swell and the breaks would lock up tight ( pump the pedal 4 times and the van would not move ) My end result had to replace all rubber lines and calipers, had to do a full system flush.

I don't know about all the rubber lines failing at once. My trucks sat for 20 years. And only one of those lines was bad when I first tried to bleed the brakes. I replaced them all anyway but I would doubt all would go bad just like that. I'm more included to think a locked up master cylinder or a clogged proportioning valve.

When the front lines on my BII swelled shut, I'd wait 5-10 minutes and I could drive it again until I used the brakes. The back never gave me a problem.

Think Proportioning valve..not brake lines.

Just reading dual piston calipers threw up a red flag to me.

The calipers did not retract once they were extended.

The heat from that has probably seized the rotor to the caliper/pads

I assume it worked before? if so, did you correctly adjust the actuator rod for the new master cylinder? if not it may be applying the brakes all the time (that would explain why cracking the bleeder released the pressure).