Any idea why my front disc brakes are holding pressure?

[Maximus]

Here is my setup... 1955 2nd Series pickup
eBay bought (dont hammer me) booster/MC kit with bracket for under floor install.
I have front stock disc brakes from the 1978 Firebird that the subframe I used was from. the rears are the same car's rear drums that were on the 10-bolt diff. that came out of the firebird.

I dont have a distribution block/combination valve or proportioning valve but I do have a 10 lb valve on the rear drums.

I had a 2lb valve up front but because of the sticking I removed it, but the sticking stays. Its only after like a mile of driving that they start to stick...
If i either of the front calipers it frees up. and after its free if i push the brakes, they release as they should.

I thought the combination valves just reduce flow to the rear drums, and thus not being connected to my pressure buildup up front.

When i put the chassis together 5 years ago I put new calipers on, so I wouldnt think its the calipers, that and if it was I would only suspect one of the calipers to have failed, not both...
That and in the last 5 years the truck just sat with plumbed brakes and they were maybe pressed 10 times?? I just finally started driving the truck within the last two weeks, around the block and such working out kinks... and then i ran into this on...

Ill try to answer any questions that may help you guys brainstorm, but I am stumped at the moment. The only thing I have left to check is to see if for some reason there is too much fluid in the MC and its somehow building pressure with nowhere to go...

any other ideas?

thanks!

Any suggestions will be considered...

[Lee H]

Sat for five years with no use, I'd have to say corrosion inside the calipers has caused the rubber o-rings to go bad causing the sticking and then allowing air to get sucked in once they free up a little. Second thought is the rubber hose (s) has/have degraded on the inside causing a restriction but don't think so.
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[Maximus]

Originally Posted by Lee H

Sat for five years with no use, I'd have to say corrosion inside the calipers has caused the rubber o-rings to go bad causing the sticking and then allowing air to get sucked in once they free up a little. Second thought is the rubber hose (s) has/have degraded on the inside causing a restriction but don't think so.
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I see what your saying, but it seems more of a master cylinder problem, because pressure builds up. If I bleed one caliper both sides free up. If the caliper was sticking I would think it would be a localized problem? no?

I am thinking of replacing my M/C or adding a prop. valve, even though I cant logically explain why that would help.

option C may be replace the whole Booster/MC assembly just to eliminate most of the parts in the system. I will try and verify that there is no way my pedal is sticking tonight...

thanks for the suggestion, and since my calipers are so cheap I may replace them too...

[maskale]

ur push rod could be to long.

The way I do it is guestimate how long it needs to be by holding the MC next to where it bolts up, see how long it needs to be. Cut it off about an 1/8" longer then I guestimate, so that I can test fit and grind off as needed. Do a little at a time, cut it to much and u will need a new rod.

There are articles that cover how to go about figuing out how long u need a push rod to be. Go goggle that. Its much to long for me to type out the process.

[Maximus]

Originally Posted by maskale

ur push rod could be to long.

The way I do it is guestimate how long it needs to be by holding the MC next to where it bolts up, see how long it needs to be. Cut it off about an 1/8" longer then I guestimate, so that I can test fit and grind off as needed. Do a little at a time, cut it to much and u will need a new rod.

There are articles that cover how to go about figuing out how long u need a push rod to be. Go goggle that. Its much to long for me to type out the process.

My Master cylinder and booster came as a mated pair, the Universal 7" booster and 1" "Corvette style" M/C If it was too long wouldnt it always stick though? Not just after a ride?

[maskale]

No. No.

I have read ur post twice, Im still not exactly sure what is exactly happening.

But, if ur brakes stick after a several stops, then u bleed and everything works great again then they stick again after a several stops then ur push rod could be to long.

[Maximus]

Originally Posted by maskale

No. No.

I have read ur post twice, Im still not exactly sure what is exactly happening.

But, if ur brakes stick after a several stops, then u bleed and everything works great again then they stick again after a several stops then ur push rod could be to long.

I think your onto something...

The external push rod TO the booster is adjustable, but I am not sure about the booster-to-MC pushrod, If i get home early enough I will pull the MC off the booster and check that rod out.

I read a little of this random thread of a similar issue, also pointing to the pushrod, but I am at work and didnt have enough time to read the resolution to that thread...

Link to Mustang forum website with similar brake issue

I read a simple test of the push rod being the issue is putting a couple washers as a spacer between the MC and booster... I think this should be safe enough to test the issue...
very simple test too.. heh.

[maskale]

On these old rides u never know what ur gonna get. I have had to cut as much as an inch off of pushrods.

So while the washer idea is a good one, and worth doing dont think u have eliminated the pushrod if after adding several washers it still isnt fixed.

If the washers work, then grind some of the pushrod off, dont leave the washer in, doesnt look professional.

[maskale]

a simple test to check if its to long,

push on the brake pedal, not fast, but not to slow, u should see some fluid come out of the ports on bottom of the MC then get drawn back in as u continue to push. Sometimes its hard to see movement, if so remove some of the fluid from the MC so that u can see the movement of fluid cause disturbance on the surface, but not so low that air gets sucked in.

If u dont see the fluid push out then sucked in the its to long, if all u see is fuild getting sucked in then it is to long.

[Maximus]

well I got under the truck and unbolted the MC. As soon as I did the MC was pushed a 1/8 inch away from the booster because of the pressure from the pushrod. I put two small washers on each stud, about 1/8", and bingo! drove around the block 3 times and no sticking!!!

thanks!

[cableguy0]

DO NOT space the master away from the booster. Some pushrods are adjustable. Check your pedal linkage as well.

[oldblue1968chevy]

I had similar problem. My soft hoses were in a bing and not letting fluid back.

Maybe pinches hoses or old hoses?

[Maximus]

Originally Posted by cableguy0

DO NOT space the master away from the booster. Some pushrods are adjustable. Check your pedal linkage as well.

The washers were to test the pressure and see if it solved my problem, now I will shorten the rod if its non-adjustable