Brake Bleeding - Floor Pedal MC

[MitchW]

Looks like things have been a bit slow here so I'll ask about a long standing issue that I've never resolved. What's the best way to bleed the brakes on a US 69 dual circuit MC car.

History: I've got a resleeved MC and I've never been able to get all the air out of the brake lines. Its not related but I've replaced the soft lines and the rear brake compensator valve, and removed the boosters. I've useed a Power Bleeder and many quarts of brake fluid. A non-Alfa shop tried pressure, vacuum and somebody pumping the brakes and finally combined all 3 with the same lousy results.

Here's a couple of tips I've heard and some I've even tried.

1. Raise the car at the front end. Haven't done this one yet but the tipster talked about using a crane!

2. First crack the lines at the MC until fresh fluid flows. Repeat at the boosters or where they used to be. Then finally bleed at the calipers starting with the right rear, left rear, right front and finally the left front.

3. Bleed the front and rear right side together and then the left front and rear. I never did quite understand how this should work.

So how is it done?

[ossodiseppia]

Have you not had any luck bleeding your brakes? Can you give us more info on the issue?

[MitchW]

Quote:

Originally posted by ossodiseppia Have you not had any luck bleeding your brakes? Can you give us more info on the issue?

[papajam]

If the pedal comes up high and hard with one pump, that pretty much rules out air being in the system. Air will usually result in a consistantly low, spongy pedal no matter how many times you pump the pedal.
Sounds like you have either caliper piston knock-back (the wheel bearings are too loose) or cal piston pull-back (the cal piston seals are hard with age and are pulling the pistons back too far into the caliper).

[MitchW]

Hard caliper seals are certainly a possibility, all the rubber in this car is dried out. Anyway to tell which caliper it might be or to be certain that's the problem? Should there be a maximum gap between the pads and the rotor?

I haven't noticed any looseness in the wheel bearings but I'll make sure to check that as well.

[Roadtrip]

I'd say if you have any doubts about the integrity of the caliper seals, you should replace them all. This would give you a chance to inspect the pistons and bores as well. What's the sense of going to the trouble of changing a MC and putting in all fresh fluid just to have to do the calipers over in the near future . . . probably.

[papajam]

After refreshing my memory by reading this little ditty in the ATE brake manual, some pull-back is normal. After looking at the pic and seeing how pull-back actually works, it would seem to me that a hardened cal piston seal would pullback LESS because it's too stiff. This contradics a tech article (without pics) that I read many years ago which I now think was incorrect. But I'm with Roadtrip on checking the calipers anyway.
I'd check the rear bearings as well including axleshaft endplay. Endplay can cause knock-back in the rear calipers.
There are two ways I can think of (there's probably more) to determine if it's the front or the rear that's the problem. The first is to get an extra brake master cylinder output fitting and weld the hole closed to make a plug. Remove the line from the M/C that goes to the rear brakes, install and bleed the plug. If the brake pedal is now high and hard, the problem is in the rear. If the pedal is soft, it's in the front. This second method will require replacement of the brake hoses when you're finished. Take a pair of visegrips or similar and squeeze closed the brakehose to the rear brakes and see what the brake pedal does. Pedal high and hard? The rears are the problem. Pedal still soft? With a second pair of visegrips, squeeze closed the brakehose to the RF caliper. How's the pedal now? Use a third pair on the LF brakehose to isolate all the calipers if necessary.

[papajam]

Also be aware of proper caliper piston alignment.

[MitchW]

Wow, great post! I appreciate the help. It looks like this will be a good weekend to check it ALL out.

Instead of blocking the hydraulic lines I could push the calipers all the way in and block them with a piece of wood to single out the calipers.

[papajam]

Excellent idea, Mitch!! Good thinking!
Which made me think of another question. Is the brake pedal high and hard with no pumping when the car is parked? Or does it required one pump only when driving?

[MitchW]

Thanks, didn't want to ruin those new rubber lines or have to mess with bleeding the brakes for each test.

It starts out low and then pumps up. It will then stay high for awhile (several uses) but eventually sinks back down if not used for awhile.