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7/8 master cylinder has no pressure initial step


DatMo

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So I've been trying to pump my brakes out and I've bled them for about 30 mins and for some god damn reason, if you leave the brakes alone for a few seconds, they lose complete pressure however if you repeatedly press the brakes, they gain about full power on the second or third push. It's on a 1972 510 with 280zx brakes in the front and steel braided lines. The rear is stock drums. I have a feeling there may be air because it's the first time this car has had fluids?

Im going to buy a speed valve to do them quicker. Any ideas what the issue could be? It's a new master cylinder as well because this happened with my 3/4 one and I thought it was the pedal or the brakes so I upgraded to 7/8. PLEASE HELP. My project is so damn close to being daily driven but shit like this keep happening :(

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Look up how to bench bleed the master to get all the air out of it. 'Bench bleeding' can also be done on the vehicle.

 

New, is no guarantee that it works. At some point you may have to entertain the idea that the master is the problem.

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I bench bleed a master cylinder this way.  Bolt it in the vehicle, take a short piece of brake line, bend a loop in it so the brake line dumps directly back in to the master cylinder.  Fill the master cylinder, pump the brakes, (or clutch) a few times, then hook up the actual brake (clutch) line.

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^^^^ and may need to push pedal slowly,not full strokes possibly too. Bench bleed until no more bubbles. Clear tubing makes it ez to see the air bubbles,but bit more pita. Also make sure mc piston returns fully and have proper pushrod/mc/pedal free play.

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If you didn't bench bleed you may have ruined the cylinder by pumping high pressure air through it.

I really hope this isn't the case cuz I'm short on money and I need this thing done.

 

As far as bench bleeding, I've looked it up and it all seems pretty simple but with the clear tubing, do you attach those to the side of the master cylinder or remove all the hardliner? I was assuming that if I just used the nipples on the side, it would be good?

 

I hope this is it. Does this pedal issue sound like its air in the system?

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Just bleed with the nips on the side.. clear tube ran to the reservoir. Pump until no bubbles. Then take the a 15 ft clear hose.. put on all 4 corners one at a time, and put end back in the respective reservoir. Pump until no bubbles. 

 

 

Bleeding complete. It's easy as shit.. and only takes one person.

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Just bleed with the nips on the side.. clear tube ran to the reservoir. Pump until no bubbles. Then take the a 15 ft clear hose.. put on all 4 corners one at a time, and put end back in the respective reservoir. Pump until no bubbles.

 

 

Bleeding complete. It's easy as shit.. and only takes one person.

I've bled countless times and never used the bench bleed method but just the two person method and it works fine. This is why this leaves me to conclude that it could be something else.
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Nope, do what I'm saying.. It takes under an hour.. and 15ft of clear hose is like $4. TRY IT. Two person method is a waste of time. If you don't bench bleed a new master, you might as well just toss it in the garbage with your old one. 

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Nope, do what I'm saying.. It takes under an hour.. and 15ft of clear hose is like $4. TRY IT. Two person method is a waste of time. If you don't bench bleed a new master, you might as well just toss it in the garbage with your old one.

Will do. I'll go buy em right now. Thanks!

I'm hoping this is the trick already dammit lol.

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My new MC cam with about six inches of clear tube for the side nipples. Bless bleed it like that first.

Then clear tubing on the brakes. Crack the bleeders open just enough. You don't need two people like dealer Draker said. The MC does not pull that much fluid back in. Just go until no bubbles in the clear hose. If you can't get rid of the bubbles your bleeder might be open too much.

 

 

Edited: Trying to fire off a quick message on the crapper at work, didnt reread before posting. Damnit.

Edited by Laecaon
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Start with your master. Bleed out the side bleeders. Once no bubbles, go around the car start with the wheel farthest away from the MC and work your way to the closest. Bleed each until there is no bubbles in the line. It's easy to do and quick since you are doing it yourself. Just make sure the clear line fits tightly around the bleeder valves.

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What about the pushrod length ? I have the same brake setup on my car.I had to make the pushrod longer,maybe a 1/4" or so if i remember correctly.

I'll do this after I bleed them but I don't see this being a problem?
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Is this the same vehicle you had trouble getting the rear brake drums on?

Yes it is. However when I bleed the brakes, the rear seem to be the ones doing just fine but the fronts seem to be acting a bit un-normal wheel bleeding them. I'm doing the nipple bleed right now though and it seems to be much better on the initial step.
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So I'm gonna try to find another friend to help me because maybe I'm missing something when I'm in the car and or maybe I didn't tighten it quick enough when I pulled off the hose but same issue! I'm gonna try to extend the pushrod a 1/4 inch and see if that helps then do it all over again.

And darker, I used the clear tubing. I'm gonna try again and if nothing, it'll be the mc.

I turned the side nipples a qtr turn when I tried it.

Damn datsuns

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Um...hose was supposed to stay on when tightening bleeder.

 

 

 

You either bled it improperly, have a bad master, or your pushrod length is bad.

 

Possibly all 3.

Gonna get a friend to help me out tonight that has some steadier hands than me so he can tighten the tube without missing the boss lol but regardless I'm gonna adjust the pedal 1/4 and see if that helps.

 

I haven't driven it to readjust the rears because the pedal has no pressure and I'm not gonna risk the car.

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No one said drive it.

 

 

When you removed the hose to tighten the bleeders, you introduced air back in to the circuit you were trying to remove air from.

 

Put the hose through the hole in your wrench for the bleeder. Try again.

 

 

 

And while you're doing all of this...just know you probably still have a bad master cylinder.

Take it back and get it exchanged. Lie to them and say you did bench bleed it. Won't hold pressure.

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