A little brake system help please...

[Oiluj]

I can't get my brakes to work propery.

I upgraded from stock 1972 brakes to maxima rear calipers, 1973 (larger) master vac, toyota 12S-8 front calipers. No other changes.

Here's the problem, pedal goes to the floor with virtually no force.

Observations:

1) Pumping brakes has no effect on pedal force. .

2) Get a "squishy" sound when pedal is pushed 3/4 travel to the floor. .

3) Rear pads are fully contacting discs. .

4) Front rotors turn easily, with maybe 0.010" clearance on both pads to rotor. .

5) No visible leakage anywhere. No decrease of fluid in master cylinder. .

6) There is no pedal force, but brakes do work when pedal is about 1/2 way down. .

7) Emergency brake works fine.

What I've done: .

1) Bench bled M/C 3 times. No air present. .

2) Bled all four corners multiple times. No air present. .

3) Adjusted vac rod and pedal clevis length. (no effect). '

4) Replaced master cylinder. (same symptoms). .

5) R & R'd master vac. Reaction disc out of place. Fixed that. (no effect).

Tomorrow I may try switching back to the stock master vac to see if that has any effect.

Other ideas?

[jmortensen]

Really sounds like reaction disk based on that description. Could it be where the disk sits is somehow screwed up and not sealing? I'd try another booster if messing with the disk doesn't have any effect.

[doradox]

Could be that one, or more, of your calipers may not be oriented correctly with respect to the disc. You are using up all your pedal travel taking up the "slack" so to speak. Caliper piston face MUST be parallel to the rotor face.

Steve

[code3z]

Are the calipers in the correct position/side? Meaning are the bleeder valves at the top of the caliper. If reversed with the valve on the bottom, no matter how much you bleed them, there is a BIG air pocket at the top. I did that mistake...once.

[Oiluj]

Thanks for the input.

The calipers are all oriented correctly, parallel to the dics with bleeders on top.

I'm hoping that going back to the old master vac solves the problem.

I'll keep you updated.

[doradox]

Are the calipers in the correct position/side? Meaning are the bleeder valves at the top of the caliper. If reversed with the valve on the bottom, no matter how much you bleed them, there is a BIG air pocket at the top. I did that mistake...once.

Sometimes even if the valves are at the top they still aren't right. The bleeder passage has to enter the piston chamber at the highest point. In the attached file the two "calipers" (sorry for the very crude artwork) on the left will bleed but the one on the right will not even though the bleeder is facing up.

[Oiluj]

Some progress, but no joy yet...

I've read some posts and it sounds like I don't need the rear pressure check valve with the rear disc brakes. Is this the case?

I removed the master vac, and used the best pushrod / reaction disc, re-assembled the larger unit and verified the master vac holds vacuum.

I still have spongy pedal, but it now will stiffen if pumped repeatedly. Guess I must have some air in the system, and it appears to be in the rear brake circuit. So now I'm wondering if the rear check vakve is inhibiting getting the air out...

Any additional thoughts?

[Ronaldo1973]

I upgraded from stock 1972 brakes to maxima rear calipers, 1973 (larger) master vac,

3) Rear pads are fully contacting discs. .

4) Front rotors turn easily, with maybe 0.010" clearance on both pads to rotor.

I think you should check the brake oil lines!

make sure that you connected the right oil reservoir with the right brake line .

since early 72 M/C has the front oil reservoir for the front wheel

while late 72 & 73 M/C has the front oil reservoir for the rear wheel

[=Enigma=]

Note that the longest brake line on the rear is actually on the drivers side so this one should be bled first. Based on this, the bleed order should be LR, RR, RF, LF.

[Oiluj]

Ronaldo & Adam,

Thanks for the input. Yep, the M/C is plumbed correctly and that's the sequence I've used for bleeding the brakes.

I'm kinda stumped at this point. I'm thinking there may be a blockage somewhere, perhaps at the rear check valve. Next step is to remove the rear check valve to see if that improves things.

If that doesn't do it, I may move the system back towards the "stock" configuration in a step-wise fashion to determine where the problem lies. Brakes on these cars are not that complicated and it just shouldn't be this difficult to get things working...

[doradox]

Are you using the stock master cylinder? Toyota calipers need more fluid volume and the stock master cylinder might not push enough.

Steve

[ktm]

Are you using the stock master cylinder? Toyota calipers need more fluid volume and the stock master cylinder might not push enough.

Steve

I switched out my master cylinder with a 1979 280zx (must be 1979 and not later) 15/16-in. unit when I upgraded to the S12/w calipers in the front and 240sx calipers in the rear.

He swapped his rear drums for rear disks as well as added the 4-piston Toyota calipers. I would suspect the master cylinder as well.

Edited September 10, 2009 by ktm