I have a 1989 pickup. Sas crawler, came with Trail Tough (not trail gear) disc converted rear kit already when i bought it. The front calipers were getting old so i swapped to the V6 and vented rotors (marlin crawler kit) when I recently rebuilt my front axle and put new diff in. It has a 1” bore MC already, i installed a Wilwood prop valve from LCE on the inlet line of the lspv which has been removed since my truck is too tall for it. The return line was capped at the T on the front passenger side wheel well. I have bled the brakes many times from longest to shortest (Driver Rear, Passenger Rear, Passenger front then Driver Front) constant stream from all lines and no bubbles or burping after first bleeds. My pedal gets completely stiff with 1 or 2 at most pumps when it is off, but when the truck is running the pedal is very soft and havnt taken it out again due to this. Any ideas? I have checked for leaks and have seen none



Have tried adjusting the valve differently while bleeding. Not solved my issue and havnt mounted yet due to brakes still having problems. Dont mind the surface rust/mud. Truck has veen down for last year with new engine, new dual cases and bunch of other needed work and this is one of the last things to have it trail ready

Used a dorman M10 plug with thread sealant after picture was taken. No leaks

Was waiting on new hardline to connect to braided lines. Installed now but picture is from 2 weeks ago


have read the other posts about the disc conversion issues and none have seem to applied or had same issue.

any recommendations for anyone who has had this issue on this setup would be greatly appreciated, thank you

 

maybe open the valve all the way up to bleed it?

Just so im understanding, you only have 1 line with fluid in it going to the rear breaks??

Did the MC go dry at all? booster boosting?

 

Not sure why you didn't just bypass the lspv and put the proportion valve at the MC. Also, do you have the correct MC with built in residual valve for the rear disc's?

 

yes i tried it like that, pictured above theirs two lines going to lspv, one inlet and one return. It Ts, i removed the return line and capped at T and theres a single line going to rear. I put the valve on that line and then after the line splits to left and righr brakes

 

I believe so, it already had the 1” bore master installed when I got the truck. I did bypass the lspv and removed from truck.

 

bump. Still havnt figured it out. Instant pressure with one pump when off. When running it has soft pedal. Dont see vacuum leak anywhere

 

That doesn't mean it's correct. Is the check valve on the booster good?

 

The pedal SHOULD feel easier with the engine running. That's what the booster is supposed to do. Less effort on the pedal. Does it stop good with the softer pedal feel?

Can you pump up the brakes and have the pedal hold solid or does it bleed down? If it bleeds down you have a leak. If it's not leaking externally (no obvious oil anywhere) it's leaking inside the master cylinder or where you can't see a leak into the booster. If you have no loss of fluid, it's not external, it is the master cylinder.

If you can pump it up and it bleeds down "SOME" but then will pump up and hold, it's air in the system compressing.

The booster should hold vacuum. Do you have a Mity-Vac to check it? The engine vacuum should hold for a couple brake applications after shut down. If it doesn't, it could be a bad check valve. Some are built in the booster and some are part of the fitting where the hose attaches.

If the booster has a bad diaphragm, you not only have a hard brake pedal, you have a vacuum leak that affects the engine if bad enough.

Keep in mind just because the master cylinder is new or newish, it can still be bad. If you can't pump up the brakes and have it hold solid and there are no leaks or loss of fluid it's probably bypassing internally. The piston pushes fluid but instead of going to apply the brakes, it leaks around the piston seal.

I hope this helps.

 

After reading your post again........Did this start AFTER the front rotor, caliper and proportioning valve change?

If so, you can separate the front and rear by plugging one off at a time and seeing how the front and rear feel. Don't drive it on the street, just for testing in the garage or driveway.

Technically you should be able to plug the output ports of the master cylinder. If it doesn't hold solid as a rock, it's bad.

This should cover everything I can think of.

 

sorry for late response. Works been busy and no time for truck. I got a new aisin master installed yesterday. Bench bleed then installed. Bled all the brakes multiple times ij right order too. The pedal is rock hard first press when it is off. When engine starts the pedal still goes to floor

i know if booster is bad it is typically the reverse: pedal gets hard when running. Ive replaced everything but the booster. Might have to just replace booster, the truck still slightly surges in rpm when pressing brakes. I was told it also might not have enough vacuum since the new engine does have a slight cam in it., but taking that with a grain of salt, since its Nothing too crazy (rv cam).

The truck is egr deleted and done right with no leaks, the main line running off the booster doesnt leak and has no cracks. It did have some debris behind it and old master, i cleaned up and when pressing the booster nipple/push rod, it releases air. So my only guess is maybe its in internal leak?

the seal had some gunk on it but i cleaned off and it looks to be intact

 

Update: started truck and pedal still soft. Pinched the line on the booster and the pedal is hard and engine rpm is correct. Letgo the pinched line and pedal is soft again and rpm fluctuates when pedal is pressed. Seems very back asswords for a typical failing booster

 

Looks like brake fluid inside the booster. Looks like the master was leaking.

If your engine fluctuates when you step on the brakes the booster is leaking. Basically a big vacuum leak when you hit the brakes. It does seem backwards but it's got issues. Time to change it. Get a dual diaphragm booster from a 90's truck. What a difference you will see.

 

Bleeders on front axle calipers should be on top. Swap from side to side and try again.

 

rsmdon, you have a good eye! I had to go back and really look at that blurry pic to see that.

 

calipers are marked driver and passenger
and fluid flow is not issue. Its vacuum related. Replacing booster today

 

Any particular reason on the front axle, why you put the calipers in front of the axle instead of the back?

 

If you mount the calipers on the front instead of the back it doesn't matter if they are marked driver/passenger, they are wrong for your application.
rsmdon hit the nail on the head, you will never get the air out if the bleeders are on the bottom of the caliper, air rises. bleeders go on top. these are facts of life.
You either need to mount them on the back side or swap them side to side with the bleeders on top.

 

Thank you all for input. Got new booster in and brakes were oriented like that when I bought and i kept same way when I recently rebuilt, unknowing it was wrong. The new booster fixed the idle issue and engine runs great. Swapped the calipers 180* to backside and rebled. Everything works great.

 

Thanks for updating this thread. Dead end threads suck. Glad you got it fixed.

I can't tell from the pic, did you upgrade to the dual diaphragm booster? They make a BIG difference.

 

It already had one it it but i think it was internally screwed. Picked this up at a local toyota junkyard and looked new. Factory JKC one.