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Caliper won’t bleed

yep just did a k5 blazer with 1tons and matching hydroboost and master from the k30 all going threw stock disc/drum k5 prop valve on the frame and stops great for me and the member here i built the k5 blazer for . he is rolling on 37" tires . build thread here K5 build he just got done with over 600 miles of road test on a trip after i did under 50 miles testing .
 
Check valve is normally in the master cylinder. I can't say that all mc have this for disc drum combo. If mc with check valve is left on a rear disc conversion, I would expect some over heating, premature rear pad wear at minimum.
 
Had a 97 Yukon do this to me once...no matter what I did, I couldn't get the rear brakes to bleed right. They had fluid, but couldn't get the air out... after about 4 quarts of fluid and two days of working on it, I had enough of a pedal that I felt like I could get it down the street to a brake shop to get it power bled.
Only time I have ever taken a vehicle to a shop to be worked on.
I built a power bleeder out of a pump up sprayer after that...
 
wasted wages said:
Had a 97 Yukon do this to me once...no matter what I did, I couldn't get the rear brakes to bleed right. They had fluid, but couldn't get the air out... after about 4 quarts of fluid and two days of working on it, I had enough of a pedal that I felt like I could get it down the street to a brake shop to get it power bled.
Only time I have ever taken a vehicle to a shop to be worked on.
I built a power bleeder out of a pump up sprayer after that...
Click to expand...


I may do this anyway. There is a shop just over a mile from my house and the fronts should serve well enough to get me there.
 
Ok thinking on this some more, I am still gonna blame the mc. 1st this is the stock 1/2 ton 10-11 inch drum brake mc? This mc only needs to move a shot glass of brake fluid to make the small wheel cylinders work.
Disc calipers need more volume to function. I know what has this to do with bleeding, well it is pressure differential, the fronts more volume build pressure, the rear doesn't and the safety valve shifts.
Even with the original mc, try jacking the rear up as high as you can safely get with your jack stands. And gravity bleed this would eliminate an air bubble.
 
chris85 said:
I may do this anyway. There is a shop just over a mile from my house and the fronts should serve well enough to get me there.
Click to expand...

my 97 of course had antilock brakes, which had a lot more tubing and a brake module which I think was full of air and caused the problem...when I changed out a rear wheel cylinder, I failed to plug the line when I ran to pick up some parts. I didn't think it was going to be a big deal, slap a new cylinder in...have someone pump the brakes and be done.... two days later and extremely frustrated, I gave up.
It gets worse, when I got to the brake shop, I walked in and explained what I needed done. The guy walks out to get the info off the truck and comes running back in yelling
"your trucks on fire!!!"
after much commotion, fire extinguishers etc, it turns out I left some rags under the master cylinder that got blown against the exhaust manifold and burnt up .. I got lucky.. minimal amount of wiring damage and the rubber boot on the steering column... and of course, my pride...:whistle::shame: :crazy:
 
I've been mad enough at my truck to let my truck burn if it caught fire more than once...was tempted to toss gas & a match at it more than once too !..(like when its 10 degrees out and snowing,and I just killed the batteries trying to get it to start!)..:angry1:

Bleeding brakes can be a real suck pill alone...I made a vacuum bleeder that works off engine vacuum,but its useless on a diesel..
I don't have any other gas powered vehicles to hook it too either..

So I have to pump up the pedal till its firm,and jam a board between the seat and brake pedal to hold it down,run back to open the bleeder & repeat several times--usually that works for me when gravity bleeding wont ..

I had tried using my shop vac to supply vacuum to the bleeder I made,but it wasn't strong enough to work..

It's even more frustrating when you have a "helper" who cant follow instructions,and they let up on the pedal before you close the bleeder screw...."Ok,push it to the floor and HOLD it"...then I see the brake lights go off,and know they didn't!..:mad2:....females seem to be the worst when it comes to this for some reason..then you end up yelling at them and they get pissed and tell you "fix it yourself"..:oops:
 
Ok, probably a dumb question but after bench bleeding how do I install the MC without leaking fluid out/introducing air?
 
After bench bleeding. put temp plugs in openings. Make sure master is full after mounting and attach lines. fluid coming out will not let air in.
 
i leave the hoses in and pull them when i install the hard lines on the vehicle .

and fyi : its rare but i just replaced a bad hose cause it was restricted / collapsed inside and not letting fluid back to the master . i have seen some rubber hoses do this over the years .

i know you got new braided hoses tho .
 
Success! Sort of.


So I finally got the rear calipers to bleed and the piston actually moves. It took a lot more bleeding than I expected. But now that I’ve had a real in depth look at everything I’m wondering if I need to change master cylinders or prop valves.

Here are my thoughts:
While bench bleeding the master cylinder I noticed the rear brakes “pressurize” first, or at the same time, but they do not continue pressurizing throughout the stroke of master cylinder as the fronts do. Also, when bleeding we did not get the same amount of pressure to the rear as the front, and this never really seemed to improve with continued bleeding. Because of this I left the bleeder tool in place for now for fear that front can still overcome the pressure of the rear and trip the switch again.

As far as I understand, the original drum brakes should have required less fluid and pressure than these disk calipers, but this prop valve should provide more fluid, but maybe the master cylinder just can’t produce that??

I could be way off base here, I’ve just been trying to wrap my brain around it for awhile. Really I’m mostly just happy that I have rear brakes now!!
 
3/16ths or 1/4 " line from master to prop valve? I am think the master.
A 1983+ P30 jb8 master might be a good choice disc disc master.
 
chris85 said:
Success! Sort of.


So I finally got the rear calipers to bleed and the piston actually moves. It took a lot more bleeding than I expected. But now that I’ve had a real in depth look at everything I’m wondering if I need to change master cylinders or prop valves.

Here are my thoughts:
While bench bleeding the master cylinder I noticed the rear brakes “pressurize” first, or at the same time, but they do not continue pressurizing throughout the stroke of master cylinder as the fronts do. Also, when bleeding we did not get the same amount of pressure to the rear as the front, and this never really seemed to improve with continued bleeding. Because of this I left the bleeder tool in place for now for fear that front can still overcome the pressure of the rear and trip the switch again.

As far as I understand, the original drum brakes should have required less fluid and pressure than these disk calipers, but this prop valve should provide more fluid, but maybe the master cylinder just can’t produce that??

I could be way off base here, I’ve just been trying to wrap my brain around it for awhile. Really I’m mostly just happy that I have rear brakes now!!
Click to expand...

The rear is supposed to have less pressure than the front. But that’s controlled by the prop valve, not the master cylinder.
 
Wes Harden said:
3/16ths or 1/4 " line from master to prop valve? I am think the master.
A 1983+ P30 jb8 master might be a good choice disc disc master.
Click to expand...

Umm cant remember the tube diameter off hand, but the rear is a 9/16” fitting and the front is 1/2”.
 
bp71k5 said:
The rear is supposed to have less pressure than the front. But that’s controlled by the prop valve, not the master cylinder.
Click to expand...

I get that, but wouldnt they still bleed with some force. When you open the rear bleeder the fluid is not very swift at all. Night and day difference between front and rear.
 
bp71k5 said:
The rear is supposed to have less pressure than the front. But that’s controlled by the prop valve, not the master cylinder.
Click to expand...
Only if the master can provide more flow/ pressure then needed the the porp valve will function.
 
Can’t remember if you replaced or checked this, but could the rubber line in the rear at the “T” be swollen closed?
 
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