1977 280z Brake pressure not hold

[240zadmire]

Hello,

 

I replaced the master cylinder because it leaked at where the booster strike the cylinder.

What I have done:

- bleed/flush the old fluid 3 times starting rear driver --> rear passenger --> front passenger --> front driver. 

Pressing on the brake pedal, I feel constant pressure from start to all the way depleted.  I had my son press as hard as he can (10 years old).  The brake seem to hold the tire, but if I try to turn the tires, I can still turn it.  It is not supper hard to turn the tire.  BTW, I'm 140lbs, average build and I don't think I'm that strong.

what I notice is that if I block the brake check valve going to the manifold, then it is really hard to press the brake.  I mean, you cannot press it at all.  I searched online to check the booster functioning using my month to blow/suck and seem to be ok.  I also check the brake check valve and seem to be ok too.   What am I missing/doing wrong here.  Please help.

 

regards,

 

[Zed Head]

3 hours ago, 240zadmire said:

I replaced the master cylinder because it leaked at where the booster strike the cylinder.

Pressing on the brake pedal, I feel constant pressure from start to all the way depleted. 

Are you saying that the brake pedal goes all the to the floor?  It's not clear what you mean by "all the way depleted".

Did the brakes work correctly before you saw the leak or are you working on a car that has been sitting for a while?

[SteveJ]

Did you ever bleed the MC?

[240zadmire]

Hi, 

apologies for not being clear.

the car was sitting for years.  Before I disassemble to work on the car, it was working.  However, a lot of dirt in the master cylinder.  I took it apart and clean it with brake cleaning aerosol.  I did not bench bleed it.  I put back and bleed the master cylinder as it mounted on the car. 
i went ahead and bleed the other drums and disk as mentioned above.  Brake very soft afterward and I notice moist on the low brake boost.  Open it up and notice wetness there.  I mean you feel the wet with your finger not dripping.  Figure this is 40+ years old, time to get a new one.

 

this time I bench bleed it before put in the car.  For measure, I bleed the master cylinder again and go about other drum and disk as mentioned above.  It is better but pressure not lock.

 

and yes, even if I press the pedal all the way to the floor.  Still not lock.

 

2 more info that I forgot.
-  I did rebuilt the front calipers and test the calipers before putting the pads in to make sure they work without leak.  The rear ones are new.

- if I pump the brake repeat couple of time fast, I notice the smaller reservoir toward the front of the engine shoot fluid up.  It is not bubble but rather like you use your straw dip into water gently blow it.

 

hope it help and hope you guys can help me

 

[Zed Head]

8 minutes ago, 240zadmire said:

2 more info that I forgot.
-  I did rebuilt the front calipers and test the calipers before putting the pads in to make sure they work without leak.  The rear ones are new.

- if I pump the brake repeat couple of time fast, I notice the smaller reservoir toward the front of the engine shoot fluid up.  It is not bubble but rather like you use your straw dip into water gently blow it.

It might be that you put the front calipers on the wrong sides.  That makes it impossible to get the air bubbles out.  The bleed screw needs to be on the top.

But, I think that the front calipers use the big reservoir.  You didn't say what year car you have.  I think that Nissan switched front and back reservoirs when they went to the 280Z.  Not sure.  Maybe you got a master cylinder for the wrong year car.

Anyway, check those bleed screws on the calipers.  Put them on the top.

[240zadmire]

Hi Zed Head,

The car is early 1977.  I think the build date was July.

you’re right the bleed screws are on the bottom and the big reservoir is for the front calipers.  Makes perfect sense as gravity will pull the fluid down no matter how you bleed it.  Only way can bleed successfully would have to hang the car bottom up to the ceiling or flip it upside down.  
I will switch calibers after kids are in bed and will update status

 

thanks

[240zadmire]

You sir or madam Zed Head, you’ve live up to your name.  I’ve switch the calipers around so that the bleed screws upward.  It works like a charm.

 

I’ve struggle for couple of days and you made it seem so simple.  Much much appreciated

 

regards

 

[Zed Head]

I had the same problem when I got mine and spent many days working on it.  The fluid squirting up in the reservoir was a big clue.

Good luck with the car.