Brake Pressure Issue

Suspension, including wheel, tire and brake.
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PoorMtnKid
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Brake Pressure Issue

Post by PoorMtnKid » 01 Aug 2020 08:36

Hello Everyone

Been a while since I have been on here, but having an issue with my brakes so coming to the best minds to try and resolve the issue. Here is my set up:
  • 280zx Front Disc
    280zx rear disc
    15/16 Master Cylinder
I went on a mission to do some maintenance on the rear cross member. I dropped the entire rear crossmember which resulted in removing the rear brake lines. After changing out the bushings, re greasing axels, replacing brake lines, etc I put it all back together. After doing a brake bleed(two party- one pumping one braking bleeder loose) I could get some pressure but only when pumping it a few times. If I step in the car and press on the brakes it goes basically to the floor. Give it 2-3 pumps and pressure builds AND holds. It does not continue to drop after pressure is built up.

After this I noticed some fluid on my transmission, turned out to be brake fluid and it was coming from the hard line that runs down the tranny tunnel to the rear. I assume it had a crack so I replaced it with a soft stainless steel braided line. After replacing this line I decided to get a vacuum bleed system. Continued to bleed the brakes as recommended(RR,LF,FR,FL) with the vacuum pump, same problem still persists. I checked the entire system and I do not see any visible leaks. I even did a "bench bleed" of the MC while in the car using the side ports and hose redirecting to the reservoirs and still the same problem is there.

The Problem : No pressure when first applying pressure. Give it 3 pumps and it has what I would consider somewhere between 1/2 and 3/4 pressure of what it has been in the past. This pressure will hold, I tested by putting a 2x4 against the pedal and seat and it stayed in place for over an hour.

Thoughts? Thanks
looking for pass. side arm rest

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bertvorgon
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Re: Brake Pressure Issue

Post by bertvorgon » 01 Aug 2020 09:30

Where is the brake bleeder on the rear caliper?

It sounds like you still have a lot of air in there.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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PoorMtnKid
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Re: Brake Pressure Issue

Post by PoorMtnKid » 01 Aug 2020 10:37

Calipers are mounted at 12 o'clock with the bleeder slightly in between 12-1.
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bertvorgon
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Re: Brake Pressure Issue

Post by bertvorgon » 01 Aug 2020 11:08

Not bad...I have taken my caliper off...get a block of wood that fits between the pistons....then you can have the bleeder totally vertical...bleed...tap with hammer to dislodge any air bubbles....pray.....

Has helped a few times, specially with a system that has had maintenance.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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PoorMtnKid
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Re: Brake Pressure Issue

Post by PoorMtnKid » 01 Aug 2020 12:03

Could this be accomplished using the vacuum method?
looking for pass. side arm rest

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bertvorgon
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Re: Brake Pressure Issue

Post by bertvorgon » 01 Aug 2020 13:04

When it comes to getting air out.....anything attempted can't be bad for the most part. Yes, the vacuum can be used the same.

Considering you say you have NO pedal until multiple pumps, you do have some significant air somewhere in the system.

If you bleed the front...does any air come out or is it solid fluid?

While I think of all my tricks, once I jacked the front of the car up to be way higher than the rear, then let a good "head" of fluid gravity flow out both bleeders, making sure the master is stayed on top of for topping up, and, that the rear is shut off as soon as solid fluid comes out.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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bertvorgon
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Re: Brake Pressure Issue

Post by bertvorgon » 01 Aug 2020 13:05

Also, who ever is pumping the brake pedal, do it nice and slow...rapid pumping can cause a type of cavitation in the master which does not help things at all.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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PoorMtnKid
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Re: Brake Pressure Issue

Post by PoorMtnKid » 02 Aug 2020 06:51

Yesterday I tried the gravity trick. Got the car higher in the front broke both rear bleeders open. Within about 2 hours it drained about .75 Qt all through the rear passenger. Barley anything came through the driver side.

While using the vacuum prior I could pull fluid through no problem. Thinking that side might have been slightly lower. I didn’t expect the fluid to flow so quickly through and it dried up. Going to try it again but not leave the garage and see what I can accomplish.
looking for pass. side arm rest

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bertvorgon
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Re: Brake Pressure Issue

Post by bertvorgon » 02 Aug 2020 08:54

Knowing that the passenger side flowed well, close THAT bleeder...which then forces the fluid to go out the drivers side. Once that flow is established, then you can do the bleed procedure.

The way the rear lines split to me, has always made a bias to one side.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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