I posted recently about my junction box replacement. I seem to have that all squared away, but I can't seem to get rid of my spongy pedal. I've bled and bled the brakes, and no bubbles come out at the calipers. Of course, while replacing the junction block all the fluid was emptied out of the master cylinder, reservoir, and brake lines. Do I need to bench bleed the master because of air that's now trapped inside?

I had a heck of a time getting a stiff pedal after some brake work. Turned out I had forgotten the third bleeder on the front brakes.

You're certain it is the brake pressure sensor junction box that measures whether we have balance or imbalance (Dash Brake failure light On).

Could you have remain air in the two rear check valves, also?

You're using a new brake master cylinder, also? If old, when bleeding brakes using the awful method of brake pedal pumping and all, and you use:
A. Second person to close the brake caliper bleeder nipple as you press down
B. Brake bleeder hose with check valve

No matter how careful you may be atmosphere can enter around the bleeder nipple through the thread interface.

A Motive Power Bleeder or similar pressure-driven method is best to bleed the brake fluid and assure bubble are driven out.

Hope that helps.

Happy Sundae.

Mac and Duffed.
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italic, bold

I have the help of my wife when I bleed brake calipers and it always worked for me.

She does 3 rapid pedal strokes and hold down on the 4th where I open rapidly a bleed plug. I figure the agitation have the bubbles in suspension long enough to get them out thru the brake lines as I get fluid out at caliper.

And for anyone else who finds this on "search" -- a piece of tape over the vent hole on the master cylinder cap will keep the fluid from coming out when you open up the system elsewhere. If air can't get into the top of the reservoir through the vent, the fluid in the reservoir won't leave. Many, many repairs (replaced calipers, front brake lines, added SS braided lines, etc.) this way and the reservoir stays completely full. Simple matter to bleed the air out of the new line, caliper, etc.

I agree that this method (taping MC Cap vent) will keep fluid in the MC reservoir, but not until the reservoir vacuum created by escaping/draining fluid stops the flow.

A more positive method to prevent fluid loss, when any/all downstream lines are open, is to block the brake pedal depressed, which prevents fluid from leaving the reservoir. Just a couple of inches is enough on pre-ABS 240 cars.

However, ABS 240s and 700/900 cars require full pedal depression, while collecting expelled fluid from a bleed valve.
[See 700/900 Brake Service Manual TP 30838/2 pages 39 and 43.]


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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Just did it on mine yesterday. Top up the reservoir as full as you can get it before starting. With very little air space in the top of the reservoir, enough vacuum to keep fluid in is created very quickly. Virtually no fluid is lost other than that in the junction block itself. I've always done it this way - much easier and, in my experience, MUCH more effective than depressing the brake pedal.

Be sure to clean top of cap with a bit of brake cleaner before putting your cute little piece of masking tape over the vent hole. When I cracked the bottom lines of the junction, NOTHING came out. And when I cracked the top lines (2 to the rear brakes), all that came out the bottom was the contents of the junction block. Cleaned up fittings, installed the new block, tightened everything down, suctioned out (turkey baster) the reservoir, topped up with new fluid, and hook up the Motive pressure bleeder to the M/C. Then - loosen the top two fittings on the junction block (rear brake lines) until you start to get a bit of fluid out of those fittings. This will force almost all the air in the new block out at that point - so you don't have to push it through the brake lines.

I bled til I got clean fluid - 2 passes all around the car. I didn't notice any air bubbles come out the calipers. And, of course, the M/C never came close to running dry. I used about 90% of 2 small bottles of brake fluid.

You can do it on the car.

Just crack open the brake line fittings at the exit ports on the master cylinder. Depress the brake pedal and hold it down with something. Tighten the fittings and let the brake pedal up. Test for pedal firmness. If it is still soft, repeat.

Basically on pedal depression, you're expelling the air that has collected at the top of the system which is hard to remove because it keeps bubbling up while the fluid is flowing down and out through the calipers. And on pedal release (with lines closed) fluid is sucked in from the MC to fill the void being created.

Wipe up (or wash off) the small amount of expelled brake fluid.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

What is different about doing this the pedal vs. using the pressure bleeder?

"What is different about doing this the pedal vs. using the pressure bleeder?"

Amount of brake fluid expelled. The air is sitting in the MC. To push it all the way through the system requires a lot of fluid and still may not get the job done as the air may be bubbling back up to the MC while the fresh fluid is flowing by it.

That is why bench bleeding is recommended to be done first. To get the air out of the MC before installation.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Put a rag under the master when doing this.. Brake fluid eats paint.
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-Matt I ♥ my ♂