Bleeding brakes... nothing comes out in front?

Alright... here's the deal. I'm working on an '87 XJ that sat for a while with the front brake lines disconnected. I put a new front axle in it... it sat for a while while I acquired parts. The front calipers are new units with no miles on them, and the front soft hoses are new as well. I'm trying to bleed the brakes, and have successfully done the rears, but cannot get any fluid to come out of the front bleeders. I've tried attaching a hose to the bleeder and put it into a bottle full of brake fluid and having an assistant pump the brakes... air bubbles come out constantly. We did this for 10 minutes pumping the brakes, bubbles non stop and no fluid.

What the heck is going on here? There are no leaks... no fluid coming out anywhere, but it seems like it's just pulling a bunch of air in. Keep in mind these are new brake calipers and thus are not clogged by rust (like the ones on the last axle I swapped in were).

Does the balancing valve need to be reset or what? The brake light is not on when we start the rig, and the master cylinder fluid level is full. The brakes on this rig worked perfectly on this rig before the axle swap.

Am I missing something here?

There maybe air trapped in the master cylinder. With the brake lines disconnected as long as it has been, you may need to bench bleed the m/c first.

Are you just "pump-bleeding," or are you using a vacuum kit? And, did you bench bleed the M/cyl?

If it sat for a while with the lines DX'd, then it's possible that the M/cyl bore has slowly drained itself and you need to bench bleed the M/cyl before you can bleed the front brakes. Since the front and rear brakes are separated within the M/cyl, test-bleeding the rears isn't a valid test.

You should be able to find instructions on bench-bleeding without too much trouble - if you're friendly with your local parts house, you can probably get the guy to dig a bench-bleeding kit out of the "cores" bin, and pick up a vacuum bleeding kit while you're there.

The problem with pump-bleeding your brakes is that if there isn't any fluid in the system, it won't get started. With vacuum bleeding, it will pull on the fluid until some comes out - as long as there's some in the system. That's why vacuum bleeding is better.

I prefer vacuum bleeding over pressure bleeding as well, simply because I have more control with vacuum bleeding and I can see if I'm running out of fluid or not (and that way, I don't end up shoving air through the lines and undoing all my fine work...)

5-90

Well, even after sitting for a while with the calipers disconnected, the fluid level in the resevoir was still almost full. The lines were tied up so they couldn't completely drain out.

I guess I will head into town tomorrow and see about finding a vacuum bleeder, we're just using the pump method, which is very frustrating for both operaters trying to coordinate.

Yeah - even with the lines tied up, they can still drain (the source of the fluid is higher than the exit - like a syphon.) The fluid will stay full, since there is nothing pushing it out. The M/cyl won't drain, but the piston bore will.

5-90

5-90 said:

Yeah - even with the lines tied up, they can still drain (the source of the fluid is higher than the exit - like a syphon.) The fluid will stay full, since there is nothing pushing it out. The M/cyl won't drain, but the piston bore will.

5-90

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I see... probably what's going on here then. There were some puddles of brake fluid under it where it sat.

Is it possible to get the fronts working again properly without doing the whole bench bleed thing IF I have a vacuum pump to suck the air out, or will I need to get/make a bench bleed kit?

Any links to info on "bench" bleeding or a summary of the process and the parts I will need?

If you can stand a little mess, the m/c doesn't need to come out. Just crack the lines loose coming off the master cyl to get the air out, while someone's running the brake pedal of course.

Sometimes hoses on caliper nipples will look like there's air present when the system's actually bled. Something else to keep in mind.

JJacobs said:

If you can stand a little mess, the m/c doesn't need to come out. Just crack the lines loose coming off the master cyl to get the air out, while someone's running the brake pedal of course.

Sometimes hoses on caliper nipples will look like there's air present when the system's actually bled. Something else to keep in mind.

Click to expand...

Alright, well I'm calling it a night for tonight. The game plan for tomorrow is to get a vacuum bleeding setup and try that first, then see about bleeding the mc in the rig if that doesn't work.

We did try it without the hose too, no fluid coming out of the bleeder when trying to bleed them like normal, and there's no pedal feel at all (goes straight to the floor), so I'm sure it wasn't just a mistake on the air bubbles with the hose in the bottle. Just getting only air.

Hmm.. could be a bad or aired up master then. Really, you might want to pull the master cyl out to bleed it. Doing it on the bench, in the vise, you will always get more bubbles out of it by tilting the thing this way and that.

On the bench, there are two ways to bleed the m/c. One uses hoses that loop into the reservoir, the other uses solid plugs into the ports.

With the hoses, run the piston at full stroke repeatedly, occasionally tilting the mc to get more air out.

With the plugs, you won't be able to get a full stroke once it starts to firm up. Little pushes will get it done, you'll be able to tell when the air's gone.

That said, at work I have used an expensive professional pressure bleeder to evacuate brake systems. Brand new m/c from the box to the car with no bench bleeding, the pressure bleeder works great. I don't expect the same from a vacuum bleeder unless it has quite a draw.

Good luck!

Well we got it working with a little bit of persistence. Went to town and bought a vacuum pump kit with a bunch of different fittings. We cracked the fittings at the master cylinder and bled them one at a time, got some bubbles out of both, then a steady stream, so must've been some air in there, then we pulled the softlines off and used one of the fittings to suck brake fluid down to the end of the hard line, then put the soft line back on and sucked the fluid down through that, then hooked the caliper back up and pulled the fluid through the caliper. Had to do that for both sides, then we re-bled all four in the proper order, and all is cool now. Brakes are working as good as they are going to get with 37's.

Now it needs a dual diaphram booster... that's next on the list.