Gravity bleed - Need for bench bleeding BMC?

[Spetz]

Hi guys,

I've recently rebuilt all 4 calipers and added a new (second hand) BMC.

After many attempts to bleed the brakes (1 person job) they still only grab right at the end of the travel.

I've put through about 1.5 quarts of brake fluid and it got a bit better but still the brakes are too weak.

I did a "bench bleed" with the BMC installed and lines running into the reservoir.
I then connected the lines and continued to do a 1 man bleed (is it possible the BMC sucked in air at this stage since the lines were empty?).

When I bleed the brakes fluid comes out throughout the pedal stroke however when driving braking action only commences right at the end of the travel.

Before I buy a new BMC what can I do?

 

[Canadastang]

2nd hand master cylinder is a bad idea IMO. I'll bet it's the culprit.

How long did the MC sit, probably dry, unused, before you bought it?

 

[andyd]

I betcha it won't gravity bleed either. If it doesn't, it is getting air from somewhere. Since I moved on from VW bugs, I don't think I've had to R+R a BMC. Anyway, was the BMC taken from an intact system or off the shelf? Tighten the fittings again. Take the MC off the fire wall. The lines should support it Carefully tilt the MC toward the fittings. Fill the reservoir. Leave the top off. Open a bleeder. Eventually it should drip.

Volume 2: I replaced all the lines on the Rat. The whole time, I kept the ABS happy by keeping the reservoir in the MC from running dry. I probably used a qt doing so. I capped the work and filled the system over night. When I replace anything, I hand tighten everything as I make up . It is the way I was taught to do it. Then go back and tighten everything in proper order. In brakes , that is working toward the wheel cylinders. After the final fit, I chased the drips all the way back

Just had a thought, Pump the pedal by hand, feel for how much play there is before you feel pressure. There may be a different push rod length.

Please post the results, TIA

 

[Spetz]

When I open the bleed nipples fluid does come out so looks like it will bleed.

There are no leaks at all. When the pedal goes to the floor the brakes engage and there is strong resistance on the pedal. If I hold it there it does not sink.

The BMC and booster were bought as a unit from a wrecked car. The rod length shouldn't be an issue. I am not sure how long they sat but the car was about 20 years old.

It feels like there is air in the system. The first few cm of travel are soft and get the brakes to grab a tiny bit (enough to just stop the car from starting to roll at the lights) and then right at the end it feels after the air is compressed the brakes start working.

Should it be this hard to bleed the system?
Is it ok to bench bleed the BMC on the car? ie, disconnect the outputs and run them into the reservoir? Then pump the pedal?

 

[andyd]

Yes bleed the BMC, then you'll have to re gravity bleed