I just finished installing a new m-cyl.
The instructions said to bench bleed the outlets, with some little
plastic dummy fittings, and short hoses back into the reservoirs.

The front brake line bled out quickly, but it took 20-30 min's to
get the rear brake line bled. Has anybody else done this operation,
and what was your result?

 

Ive done that many time at the shop as long as the line go back into the reservoir that will stop it from sucking back are then you should be good. the rest is easy to bleed each wheel

 

Last time I bench bleed I didn't have much luck with that method. This was on a M/C without the residual pressure check valves.

It would push air/fluid out but then pull it right back in. Ended up having to crimp the line with my fingers prior to releasing the piston. That helped but still didn't work all that well.

The best way and the method I finally used is little known. You plug both outlets so no fluid comes out.

Then tip the front of the M/C down about 30* from horizontal and bleed the air out via the compensation ports. This is done by gently depressing the piston. Don't go too far on a used M/C as this will damage the seals (don't bottom it out).

Once the M/C goes solid, IOW, can't push the piston in more then about 1/16", you are good to go.

Leave the plugs in place until the M/C is back on the car. Once the lines are hooked up I bleed the calipers via a MityVac and a catch jar. Using vacuum to 'pull' the fluid through the lines.

RBob.

 

Rbob:

Was this one or both outlets? For the rear, I configured the dummy line
so that the fluid drained back to the port, while the air bubble moved
on up the line. It took a long time, but eventually no more air bubbles
were expelled. So my question was if others had this experience, was
it unique to the rear line chamber, and did it take much longer than
the front line chamber?

PS

You still producing the EBL units? That's next on the list, after I get
some other little gremlins exterminated.



kk

 

It was both outlets. Neither one had a residual pressure check valve. And neither one was cooperating. With the valves in place this method of bleeding works. BTDT in the past on drum/drum M/C's.

With your M/C it may be that there is a check valve in the front as most of our 3rd gens have rear drum brakes. {edit: noticed the disc rear in your mini-sig. So there shouldn't be a check valve in that M/C.}

Without the valves it was a no-go. The other method I outlined worked well and quickly. Didn't take more then a minute to bleed all of the air out. Took longer to make up some plugs for the ports.

Yes to the EBL Flash. There is also the EBL P4 Flash for those with '90 - '92 TPI set ups.

RBob.

 

Good info

The rear disc is from an axle switch, rather than the original equipment.
And I didn't specify rear disc when purchasing this M/C.
I've eliminated the OEM prop-valve, with an adjustable valve for the
rear line, since I want to tune it my driving preferences.

So how important is the check valve for disc operation?

 

A residual pressure check valve (RPCV) isn't used for disc brakes. That is unless the M/C is below the level of the calipers. In that case a 2# check valve is used. Drum brakes use a 10# check valve.

If the rear brakes are dragging then likely a check valve is in the M/C. If the rear brakes don't drag then most likely there isn't a check valve in the M/C. As a 10# check valve will cause disc brakes to drag.

Back in the drum/drum days the check valve was located behind the outlet seat in the M/C. The seat was brass and could easily be pulled out. Then remove the check valve and press the seat back in. The valve being a smaller rubber boot and a spring.

Those M/C's were also cast iron, not aluminum.

The '92 FSM breakdown of the M/C does not show any RPCV. Unfortunately it doesn't show the break down of the combo valve. As GM may have placed it there. And with it gone no need to worry about it.

Looking at the M/C breakdown I recalled it being a quick take up design. This may have had something to do with the difficulty in bench bleeding it.

RBob.

 

This is how I bench bleed, and quickly (less than a minute). Sounds complicated but when you get the general idea it works well.

Fill the resevoir with fluid and install the cap. Hold the cylinder level with the piston facing you. Place a socket (to engage piston) with an extension between the piston and your hip bone, and hold the cylinder over a drain pan. Wearing latex gloves, plug the ports with your fingers. Depress the piston by pulling the cylinder toward you. As you feel pressure on the first port, allow fluid to spray out - keep the second port plugged. Stop pushing the piston in, and plug the port with your finger. Release piston pressure. Repeat 2-3 times for first port. Repeat the procedure for the second port, keeping the first port plugged at all times. Keep in mind the second port will require more force and farther travel on the piston to push fluid out.

 

Marc:

Seriously -- thanks for the input. Its always great to hear how
others solve these problems.

Less than serious -- I hope you did this in the privacy of a secure
garage, lest anyone get the wrong impression.

kk

 

I do this stuff for a living. Bench bleeding the traditional way is time consuming, messy, and still requires the master cylinder to be bled on car. All 3 are bad for my paycheck.

 

I don't get this bench bleeding crap. Can't vacuum do the same thing? I don't get the whole tube in a container full of brake fluid thing. Bleeder fittings never seal on the threads, so you're going to just suck up air. I don't get the manual's way of bleeding the master cylinder. It says to push in the brake with the fitting at the cylinder cracked, tighten the fitting, and then let off the pedal. If there is air in the lines, isn't it going to get sucked right back up into the cylinder? I don't even get how you can tell if air is bled from one of those fittings.

 

its very easy to nech bleed a new master, put it so the piston is horizontal and clamp in a vise, take the lines from the master to prop valve and sttach them to the master, get some clear tubing and put that over the lines and back into the master resoviour. fill it with fluid, push the piston in/out untill no more bubbles. if you have problems your doing something wrong - I have done it 3 times now

 

I accidentally had the lid off my master cylinder and pressed the brakes and fluid flew everywhere...so I don't see how this is done without making a huge mess.

 

You close the line (bleeder on caliper or brake line nut in master cylinder) before the fluid pressure is released. That way air can leave but not get back in.

 

My suggestion is to just do it before you claim the process is bogus. You are pushing the piston manually. If you go slow and steady all the fluid remains in the reservoir. It is way easier to make a mess with the brake pedal since:

1. you have much greater leverage
2. you are using your leg instead of your arms, more strength
3. you may also have vacuum assist (if engine on)

 

I don't see how if there is air in the actual brake lines that it won't make it back into the cylinder since gases are lighter than liquids.

 

Maybe ask this guy.
http://www.huffingtonpost.com/2013/0...n_3428202.html

As you point out, air is lighter than brake fluid and indeed rises. Which is exactly the issue here. As installed, there is a local high point in the system at the tip of the master cylinder, among other places. This is where the air gets trapped.

 

I'm not getting how the lines going into the master cylinder would trap air bubbles and not let them pass to the cylinder.

 

Perhaps create a new thread so we don't bog this one down.

John

 

the key is that the piston in the master MUST be horizontal, it's not when it in the car so put it in a vise or jack the rear end up

 

Agreed.

 

Just had to thank this guy for this idea. I live in apartments now, and a bench bleed just isn't possible. I also can't stand in the parking lot while squirting fluid into a pan. I've been debating taking it to a shop just to pay for the hour of work for a week now.

I know it seems obvious, but I'm spoiled and have always had a decent place to work on vehicles before I moved here, so I'm not that creative I guess, lol.