New Master Cyl - won't bleed ...?

[GeoffT]

I know this isn't rocket science, but I'm stumped. Here's the situation:

- new 7/8" Tokico master cylinder (from Courtesy Nissan)

- rebuilt brake booster (from MSA)

- new shoes and drums

- silicone brake fluid

I mounted the MC on the car without bench bleeding, connected the brake lines, filled both reservoirs, and by pumping the brake was easily able to push fluid from the front reservoir through the MC front bleed valve. But could get no fluid through the MC rear bleed valve or to the rear line when I loosened it.

Took the MC off the booster and placed (gently) in a bench vise. Using a steel punch (in place of the booster pushrod), was able to push fluid through the rear bleed valve. Fluid also dripped from both front and rear line ports.

Checked the booster pushrod - it appears to protrude about 3 to 4 mm past the MC mounting flange, which I think is correct.

Put the MC back on the car, refilled the reservoirs, pumped the brakes. Again, good fluid pressure from the front reservoir - not even a drip from the rear bleed valve!!! What the hell am I doing wrong?

If you can offer any advice or direct me to any information sources, I would greatly appreciate it. I have looked at many brake-related posts on this site.

Thanks - Geoff T.

[JimmyZ]

The following are just a theorys since I can't find my manual right now. It also does not explain why only the front works but it is worth a try... One theory is that rear circuit may not be moving/retracting far enough. This would not allow it to "gulp" any new fluid from the reservoir. Do you see fluid movement when you pump?Make sure that the piston is not binding upon retraction and try screwing in the booster screw to a shallower depth. An improperly applied check valve in the MC comes to mind but your bench bleeding ruled this one out.

Another theory is that a bleed point in your rear line is clogged. Removing the bleeder and checking for flow using air might help here. Bleeders can clog. Although extremely rare, flexible hoses sometimes rupture internally and close off. This could also give a no flow situation.

Try pressure bleeding it. For starters, remove the brake line that goes to the MC rear circuit and open one of the rear bleed valves. Blow 10psi or so through and verify flow.

Assemble MC to its lines make a cap which can be pressed over the reservoir and accept an air gun. Fill the MC half to 3/4, wear goggles, crack a rear bleed valve and apply light air pressure with your cap to force the fluid through. When applying air pressure do not exceed 15psi. Start with 10 or less and use a rag to limit splatter if there is to be any. Before removing pressure cap from reservoir allow a couple seconds to pass so that the air escapes. This way you won't get a big slosh of fluid all over.

As I mentioned earlier these are just theorys of mine. Wish I could find a sectioned drawing of the MC's guts. That would help.

Oh yeah, is all vaccum removed from the booster. I know you probably know better than to start an engine during bleeding but it's another thought.

[a7dz]

You named your problem. The rod should be 3-4 mm short of the aluminum mounting flange or 3 mm taller then just the brake booster. Had the same problem on another Z last week end readjusted the rod and all is well.

[GeoffT]

Guys - thanks very much for your responses. It sounds like the booster pushrod is the likely problem. My '73 Clymer service manual is not very detailed on this aspect. It does use a caution note to state that the pushrod should be facing up when making this adjustment ... does this sound right?

Thanks! -Geoff

[Arne]

After having heard of this issue several times lately, I'm going to have to assume that the third-party service manuals (Haynes, Clymer, and the like) must all have very poor descriptions about adjusting the booster pushrod. The FSM has better instructions, as well as a decent cut-away diagram, IIRC. (Out of town today in a motel room. Can't look at my FSM to be sure.)

[a7dz]

After having heard of this issue several times lately, I'm going to have to assume that the third-party service manuals (Haynes, Clymer, and the like) must all have very poor descriptions about adjusting the booster pushrod. The FSM has better instructions, as well as a decent cut-away diagram, IIRC. (Out of town today in a motel room. Can't look at my FSM to be sure.)

I agree Arne and can say the third party manuals are confusing at best. May be better to take pictures of one being set up correctly and Post it as a tech article here.

[GeoffT]

It was the booster pushrood. I adjusted it to about 3 mm out of the booster and the bleeding process went smoothly. Thanks!! - Geoff

[Arne]

Related note - I installed a new (proper early-style) master cylinder on my red car today. The previous owner had a shop replace the cylinder last spring and they installed the later-style cylinder on it, I wanted to get back to stock. I found that to get the later cylinder to work on an early booster, the shop had to extend the pushrod length as far as it would go, and also readjust the pedal pushrod as well. Both of those changes had to be undone for me to return to the original type of master cylinder.