front brakes building and holding pressure after you release pedal???

[FUMMINS1997]

installed a 79 f150 44 and 9".. everything went together good.. installed all new brake components except the H block and line running along the frame.. absolutely everything else is new.. new booster, master cylinder, prop valve, and all calipers and cylinders.. hoses and lines.
So now my problem. the front brakes only retain pressure... to the point of having to manually bleed off pressure just to be able to move.. whats my issue. like i said everything is new..

 

[mrdrnac]

Need to adjust the push rod that connects the booster to the master cylinder, it sounds like it is adjusted too long.

 

[FUMMINS1997]

yes holding pressure after letting off pedal... and i tried adjusting the pushrod between the master and booster.. it was adjustable... but that sum bitch wouldn't move... i figured that could be a potential problem point.. so yesterday i shimmed the master away from the booster with a couple washers on each side.. just for testing purposes.. didnt seem to make any difference.

 

[FUMMINS1997]

not sure if this needs said or not but its a 73 thathat was previously manual 4 wheel drum.

 

[EPB72]

Now with brakes in fault mode ,,loosen master cylinder from booster does the issue go away .loosen till you know there 's freeplay....

You also say new proportioning valve OE style or adjustable rear line only?? you also say H block ,I'd think if its a Oe style prop valve you 'd eliminate the H block May not be the issue but elaborate more on how it's plumbed pic's could help...

 

[FUMMINS1997]

oe style.. but i ran the lines from the new prop valve to factory H block.

 

[FUMMINS1997]

when i assembled the new booster and prop valve i was unable to adjust the rod.. it just flat ass wouldn't spin.. i was afraid id bust it.. so i installed it as is..

 

[mrdrnac]

Your push rod between the booster and the master has about 1" of adjustment so it could be adjusted way too long for your master cylinder spacing. They make a neat tool to set the push rod length. I have atteched 2 pictures to help explain.

 

[FUMMINS1997]

thanks.. guess ill pull it all apart tomorrow

 

[DirtDonk]

Do the test mentioned first, before pulling it all apart. Don't chase your tail if you don't have to.

Things that can do this are:

1. Old or defective soft hose.
2. Stuck caliper(s) (less likely to be both of them, but not impossible)
3. Rod adjustment
4. Bad booster release valve
5. Bad combination (proportioning) valve.
6. Bad master cylinder.
And no, it makes zero difference that most, or all of these things are new. New does not mean good anymore.
Hence the need for testing to narrow it down.

As mentioned, the next time they're sticking (stay close to home of course!) start loosening things to see where the pressure is being held. Start at the top at the master-to-booster interface. Then just keep working your way down until you find where it stops.

Paul

 

[DirtDonk]

And by "stuck caliper" I mean with most calipers, Ford included, it could be a sticking seal and/or piston, OR it could be that the caliper sliding surfaces at the brackets are not sliding smoothly.

By that same token, if the caliper sliding bars were installed wrong, or were the wrong parts, they can jam the calipers into the brackets.

Also, what about a residual pressure check valve? Very few modern master cylinder replacements come with them anymore, but it's not unheard of I don't think.
What master cylinder are you using?

Paul

 

[FUMMINS1997]

did some testing this morning and im 99.9% sure it was all in the pushrod adjustment.. i was worried id bust it but i just got on it with a bigger wrench and is broke free and i must have taken a half an inch out of the total length.. seem to have made all the difference.

 

[Jedeka]

Glad you may have found the issue. Thanks for the update.

 

[mrdrnac]

Great news, you can fine tune it as needed, in most cases a small air gap master cylinder to pushrod works best, .020 or so, just my experience.

 

[DirtDonk]

Great to hear!
Don't forget there is such a thing as too much gap too. So if you notice the pedal has more travel than you remember (or happen like) there may be a bigger gap than necessary.
With the pedal arm ratio added into the mix, if you have for example a 1/8" gap, you could have at least 3/4" extra travel at the pedal depending on the pedal arm ratio. So no foul in keeping the gap small.
Just not too small, as you found out!

There should be an obvious clue when this happens though. You didn't mention it, but the master should have bottomed out on the rod before the mounting flange bottomed out on the booster. You would have felt pressure and had to push the master inward in order for it to seat.
The nuts would have easily pulled it inward with little feel, but prior to the nuts there should have been some resistance to the two components mating up.
A master should drop right on to a booster with little to no resistance.

Did you feel that either in the beginning, or while messing about with it after the problem started? If not, then maybe it's not always something that can be detected after all and we may have to start recommending measuring every time no matter what.

Enjoy the good brakes!

Paul

 

[FUMMINS1997]

definitely some fine tuning to be done.. but stoked i got my problem solved. thanks everyone