Jeep LJ, replaced calipers on all four corners. I already bleed the front calipers and they bled as expected.
While I was waiting for the correct rear calipers - long story - one of the rear calipers gravity bled as I forgot that I had left the banjo bolt slightly cracked. Either way, fluid seemed to make it to the back calipers.
If I crack open one or both rear caliper bleed screws, and then try to bleed the brakes, there is barely any brake pedal movement - I hit the pedal, it gets to the point where pressure builds up to the front calipers and the pedal goes hard.
If I crack open one front bleeder screw, I get the expected soft pedal. I only have a single bleed tool right now so I can't test if I can bleed the rear brakes with one of the two front caliper bleeders cracked open, but it seems odd that that's the case.
I can't gravity bleed the whole system as I've now got Speed Bleeders at all four corners, but I left one of the rear bleed screws open with the regular bleed screw in it to see if I get any gravity bleed effect. Nada.
Other than opening up the various connection points for the rear brake line one by one and try to locate the blockage this way, is there any better way to track down what the issue is? Anybody seen this before?
Workshop manual basically says to gravity bleed the brakes by having all four bleeders open, then go to the standard manual procedure, aka pump the pedal. Truck has no ABS.
Try looping some hose from the rear output on the master back up to the reservoir, and "bench bleed" it, even though it is already installed. All kinds of weird E36 M3 can happen when the MC goes dry. That's all I got.
Looks like I found the culprit. The front outlet of the master cylinder - which does appear to feed the rear brakes via the brake proportioning valve/splitter - is bone dry when I disconnect the metal brake line. I actually undid both outlets at the master cylinder and the rear one nicely spurts out brake fluid while the front remains dry. Reconnecting the rear master cylinder outlet only and it immediately went back to the hard pedal, even with the front outlet still open.
I guess I very likely have a bad master cylinder then. Ah well. Better get some bench bleeding equipment then.
<goes to reload the parts cannon>
Is this a new master cylinder? I would think this is a pushrod length issue. Measure the puahrods length versus the gap or cavity in the master. I'm guessing the port is never being uncovered because the pushrod is too long.
No, this is the original master cylinder on the truck. Looks OEM (and slightly neglected, like the rest of the truck).
It could well be a pushrod issue, but I think I'm the first person who ever disconnected the master cylinder. Either that, or it was replaced quite a while ago.
The rear brake lines had fluid in them when I disconnected the calipers. My suspicion right now is that it's internal corrosion on the master cylinder that's causing this problem.
Sounds like the proportioning/combination valve is stuck.
You may need to leave one of the front bleeders cracked to eliminate the pressure bias while bleeding the rears to get the valve to reset.
Run a piece of tube from the front bleeder into a bottle of brake fluid and so you don't pull in air.
Bleed up the rears using the two person method until the valve centers and fluid flow to the rear brakes is restored. Once the flow to the rears is restored I would close the front and bleed up the rears before going back to check the fronts.
I just saw the new post, try cracking the lines where they enter the combination valve from the master to see if it allows the pedal to move and determine if it is the master cylinder or proportioning valve.
Sorry, the photos are still on the phone, that makes is a little hard to explain.
I cracked both hardlines between the master cylinder and the combination valve at the master cylinder itself and verified that I have brake fluid at the master cylinder's rear outlet, but not the front one. That's with the pedal going through a couple of cycles of full travel. So basically, no fluid is getting to the combination valve's connection for what looks like the line to the rear brakes. I also did verify the line to the rear brakes is bone dry at the combination valve.
In reply to BoxheadTim :
Turn the regulator down real low on your air compressor (like lower than 10 psi), remove hard line from MC port that isn't flowing fluid, and gently blow air in through that outlet port, and watch for bubbles in the MC reservoir. Seems like there's something physically blocking that port.
In reply to AngryCorvair :
Thanks - I'll try to give that a shot, too. I did try achieve the same with a bit of a reverse flush using brake cleaner, but that didn't work.
Although now that I've rebled the front brakes and the fronts are working well enough to squeal the tires (yay Hawk Street pads), I have a soft pedal. Makes me wonder if I dislodged whatever was blocking the port. But that'll have to wait a couple of weeks for me to look into.
You stepped on the brake pedal too far when you had air in the system. You jammed the front piston on a rust ridge, and now it won't come back enough.
TL,DR: When bleeding brakes, never push the brake pedal farther than it normally travelled before opening any bleeder. unless the cylinder is brand new.
Streetwiseguy said:.
TL,DR: When bleeding brakes, never push the brake pedal farther than it normally travelled before opening any bleeder. unless the cylinder is brand new.
Another reason gravity bleed is truly FTMFW.
In reply to Streetwiseguy :
That's probably what happened, although I'm usually careful not to do that. OTOH, based on how crusty the calipers were and how original the brake fluid, there is a good chance that any slightly longer stroke than usual could've triggered this issue.
Oddly enough, now that I've re-bled the front brakes after disconnecting the master cylinder, I have a soft pedal so I may have accidentally unwedged the piston. Anyway, it'll be a couple of weeks until I have time to play with the truck again, let's see if it unwedged itself and/or if a new master cylinder is adviseable.
AngryCorvair said:
Streetwiseguy said:.
TL,DR: When bleeding brakes, never push the brake pedal farther than it normally travelled before opening any bleeder. unless the cylinder is brand new.
Another reason gravity bleed is truly FTMFW.
Gravity bleed doesn't work with Speed Bleeders though, and I haven't found any other decent one person bleed solution.
A hose in a jar of brake fluid will do just as well as a speed bleeder. Nasal canula oxygen hose is perfect.
