Pulling in air somehow

[Gyratingyak]

Hey fellas, I posted a shile back about a lousy pedal, I found a loose fitting and swapped my drum distribution block for a dr.diff 73 abody disc/drum prop valve. I recently did a front disc conversion with a Dr.Diff 11.75” brake kit and his stock a body dist/prop valve and used a Right Stuff 15/16” bore MC. I had to make all new lines because they were all siezed in my 50 year old drum distribution block. Everything bled great and I had a great pedal. Then when I went to bed the pads, after 4-5 hard stops from 60mph to around 15mph the pedal goes limp noodle and spongy and needs pumped up. I rechecked all my lines, found 1 tiny fluid weep at the front inlet to the prop/dist block from the MC and tightened it, rebled and tried again, same deal, but no fluid leaks this time, anywhere. Dry as a bone. This only seems to happen on hard braking when you push it excessively hard like a panic brake situation, where it is depressed rather far. If it is just easy stop and go, my pedal feels fine, but on a hard, quick stop it needs rebled again. I think it might be my MC, but I really hope not because it’ll be a pain to remove since its the bolts that hold the pedal assembly on too, I might go and buy the 2 to 4 bolt adapter if I have to do it again. Any tips from the gurus on here? I’m at my wits end and just want to drive the old girl with her all new suspension with adjustable everything and boxed LCAs. Below are some images of my baby, hopefully I can figure my problem soon. Thanks in advance for any replies, school and work have me busy as hell and its hard to find time to wrench. So if I can narrow it down it’ll help a lot.

 

[PP1RT]

My vote is on the master cylinder being bad.

 

[Gold Rush]

Before you go through the trouble to remove the MC, find some plugs to block off the outputs then stand on the pedal. IF it leaks down you will know but if it stays up solid that should tell you the MC is OK. I recently accused my MC but it passed the test. Found the REAL problem was I had installed the calipers wrong.

 

[Bobo]

Are the bleeders at the top of the calipers? They should be .. just trying to help

 

[WileERobby]

What about the rear axle block, lines, and wheel cylinders ?

 

[Gyratingyak]

Are the bleeders at the top of the calipers? They should be .. just trying to help

The doctor diff calipers have bleeders on bottom and top. I bled both just in case.

 

[Gyratingyak]

What about the rear axle block, lines, and wheel cylinders ?

The rear circuit shows no signs of air when I rebleed, but I do it anyway as a precaution. The rubber line to the block is new, but my original block was in good shape and nothing was siezed so I kept my original hardline due to then looking great and T block. I bought an extra before the swap just in case it was going to be difficult like the distribution block

 

[Johnpat]

Sounds like fluid bypassing piston in M/C
with hard push

 

[Gyratingyak]

Sounds like fluid bypassing piston in M/C
with hard push

Just a bad MC then?

 

[Johnpat]

Yep

 

[Johnpat]

Maybe pedal adjustment,pushing piston in
To far?????

 

[Gyratingyak]

Maybe pedal adjustment,pushing piston in
To far?????

I didn’t know that was a thing because I bench bled it using full stroke and there is a definite stop at full travel. Maybe it sat too long on the shelf and seals dried out

 

[Johnpat]

Pedal problem is very seldom a problem like yours,
it was just a thought.
Still go with seal in M/C bad

 

[Gyratingyak]

Anyone know the size the ports are on the aluminum 15/16 dr diff MC? 9/16x20 and 1/2x20?

 

[///Matt]

Did you use brand new fluid, or a bottle you’ve had open a while? Brake fluid takes moisture out of open air very readily. Moisture in the fluid gets hot and evaporates and introduces squishy gas to the system that wasn’t there before.

 

[Gyratingyak]

Fresh fluid.