Front Discs/MC problem (2 Viewers)

[Mainer7]

Hey everyone, I've got a mystery problem with the brakes on my '78 40.
Some history...

-original MC was leaking and replaced with a new OEM unit from Toyota Dealership.

-After about 2 years of daily use, one day I go down the road, as usual, need to slow down (going about 30mph.) I barely even touched the pedal and my front brakes immediately locked up, squeeled, and scared the crap out of me! Jacked it up and the fronts were locked, but rears were turning as they always had. Managed to crawl back 300 feet to my draiveway....I've driven the rig with great brakes for the past 6-7 years, up until the first MC failure (leaking.)

So, since then, here's what I've done...

1-replaced mc
2-replaced front calipers, pads.
3-bled, bled, bled, and more bleeding.
4-checked for excessive exhaust heat near any brake lines

Here are things I have not done...

1-replaced '78 booster (seems fine, no vac. or brake fluid leaks.)
2-adjusted push rod
3-adjust rear drums
4-replace all brake lines (no visible leaks, ballooning anywhere.)

Some more info...
currently no E-brake or any of the oem E-brake setup (drum, etc.) because now it's got an H55F, etc.

This is where it gets interesting.
Now, start it up and just going back and forth enough to hit the brakes 8-10 times, and the fronts lock up TIGHTLY! The day before, I was driving it like normal, and the next morning, surprise! WTF!?

I pop the hood and crack the FRONT brake's hard line where it enters the MC
and immediately I can hear the pressure release and obviously see/feel it happen, too. It seems to keep doing this indefinitely, and before they lock up, they are so touchy I'd likely go into the dash if I used the pedal "normally."

Myself, and my mechanic have bled this thing PLENTY.
There are no leaks I can see in the lines. Pressure is building up, returning to the MC, but not releasing. The MC is from Advance Auto and pretty much looks just like my OEM one.

Here are some possiblities we've come up with so far...

1-faulty MC from Advance Auto?
2-faulty booster?

I'm not positive, but it really seems to be isolated to the front brakes, they are the only ones locking up, etc. Has anyone been through something like this before or have any ideas about what is wrong? I searched, but after about an hour of reading posts that didn't really match, I decided to just ask...

Thanks for reading

JR

 

[WES1977]

One your master cylinder do you have 2 resevors? Which brake line do you have hooked up to the front inlet(front or rear brakes)?

 

[Coolerman]

I would start by following the procedure in the FSM for adjusting the pushrod between the master and booster. Just as a test adjust it in a bit then try it. If it fixes the issue then adjust as per factory.
Another remote possibility is the push rod from the pedal to the booster and the pedal to floor height.

Good luck!

 

[Mainer7]

wes1977, I have the 2 reservoirs, like my original was, too.

Coolerman, I will try this out. i did not try yet because I never messed with it ever. Are these adjustements normally changing by themselves? I mean, why would these make my front brakes lock up all of a sudden, one day?
Thanks, I am not a brake specialist so excuse me is my assumption is ignorant. This is so frustrating, I can do a 24v diesel swap, but can't figure out the front brakes!?

Thanks guys


edit: wes1977, I have the line hooked up the same as they have always been. I only changed the Master. Front line is going to port furthest from booster. thanks.

 

[WES1977]

ok just making sure.

 

[dgangle]

I'd bet $$ adjusting the pushrod would remedy this. I don't know why it changed and this is most likely a question noone can or will answer.

 

[Mainer7]

edit: recently tried adjusting pushrod and that was not it

Have a new MC going in tommorrow, hope it was just a bad one..
Thanks again,
JR

 

[cruisernica]

hey that sound to me like the calipers are geting stuck probably dirty with rust iside or debris rebuild it have them hone .

 

[Coolerman]

edit: recently tried adjusting pushrod and that was not it

Have a new MC going in tommorrow, hope it was just a bad one..
Thanks again,
JR

Bummer, I thought that would be it. Another thought: Any chance they gave you a drum brake master instead of a disk brake master?

 

[Mainer7]

Update: The problem happened originally with my OEM master, and since then, I'm mostly sure it was never the master that caused the problem. Took off the master today and while it was off, started engine up and pressed brake pedal. Seemed to be okay on first pump, but got progressivly harder. So this makes me say, the booster is toasted. It should be freely/easily pumped when the master is not attached, right? Well, have a new booster coming in a couple of days.
Thanks again for all the ideas.

 

[Fast Eddy]

My OEM Toyota front MC circuit had a residual valve in it. My theory is that there were plenty of late '70s applications that had front drums, and they only have one part number, and it comes with the residual valve in both circuits.

Open the front circuit, check for the valve (sandwiched plastic and metal discs), remove it if it's there, rebleed MC and front brakes, and you'll be set.

The rear brakes have to be adjusted for the whole system to work, or even to get the front circuit to bleed properly. I wouldn't even start on the front until the rear was known-perfect.

 

[Mainer7]

Fast Eddy,

Thanks for the tips, I'll be sure to make sure the rears are adjusted, too.
About the residual valve, yeah, my OEM master had that, i think. It was like you said, but had a tiny spring between the "discs." I must re-iterate, that the problem happened "out of the blue" one day. I'd been daily driving it with the OEM master for 1-1.5 years and then one day %&**&!!! This also makes me think it's not the master, but the booster. There must have been a contamination of the booster from the bad (original 1978) master that was leaking between the 2.

Thank you!
JR

 

[Mainer7]

Thanks for all your ideas guys! I got a booster from cruiserparts dot net and that totally fixed the problem. Well, at least now i have a nearly totally new brake system...
Best regards, JR