Brake Booster/Master Issue?

[Ninjaneer11]

I just had my front brake rotors resurfaced due to some pulsating while braking. Disassembly requires disconnecting the hydraulic line from each caliper on my setup (older wilwood arizona z car kit). I put everything back together with fresh pads and flat rotors this weekend and started bleeding the lines, but I hear an air "breathing" noise when pumping the petal. Here is a video of me pumping the brakes https://imgur.com/a/w5kBNBj. Please excuse my fumbling with the pedals, it's hard to video and pump the brakes while standing on one leg. The brake petal also won't stay firm. It will firm up when you pump it, but get soft after a bit of doing nothing. The petal was firm and worked great before this rotor project. I have:

• "bench" bleed the master (on the car).
• gravity bleed the lines.
• used a one-man vacuum bleeder.
• had my wife help me do the ol 2 person bleeding procedure on all 4 corners and I still get this noise.

This isn't my first time bleeding brakes and have done them successfully on this car before. Other things I checked/tried:

• It's not leaking fluid anywhere. 
• There are no bubbles in the lines when I bleed them.
• I checked and tightened the vacuum line to the booster.
• Replaced the check valve with a new one I had in my parts stash with no luck.

Has anyone seen this before? I'm assuming it's a bad booster or master, but was hoping for some advise before I start ordering parts. Could draining the master have created a leak due to old seals or something? It looks like a booster would have to be rebuilt too since they are not available. I have a 7" early 240 style booster with a 15/16 MC. It's driving season where I live and am hoping to get back on the road asap!

[heyitsrama]

When the motor is on and you press the brake pedal does the engine RPM change? 

power brake exchange in San Jose California might be able to rebuild it, they repaired my 74 booster, have them powder coat it too. 

[Ninjaneer11]

1 hour ago, heyitsrama said:

When the motor is on and you press the brake pedal does the engine RPM change? 

power brake exchange in San Jose California might be able to rebuild it, they repaired my 74 booster, have them powder coat it too. 

I just tried this. No change to rpm no matter what I do to the brake petal. This was in the garage starting it with the choke engaged, not sure if that matters or not.

[heyitsrama]

Usually when the boosters fail they cause the RPM change when you depress the pedal. 

Thats a lot of air movement in the video, can you feel where it escapes into the atmosphere?

[Zed Head]

5 hours ago, Ninjaneer11 said:

Could draining the master have created a leak due to old seals or something?

It's pretty well described around the internet that master cylinders can fail while bleeding them.  The ends of the bores never get used during normal fully-bled operation so they tend to corrode.  If you push the pedal through a full stroke you push the seal down to the corroded area.

If the pedal sinks with your foot pressure on it that's a sign of leaking seals.  

The other possibility for never getting a high hard pedal is that you accidentally switched the calipers when you had them off and you put the bleed screws on the bottom.  Not uncommon.

[Ninjaneer11]

12 minutes ago, heyitsrama said:

Usually when the boosters fail they cause the RPM change when you depress the pedal. 

Thats a lot of air movement in the video, can you feel where it escapes into the atmosphere?

Good to know. I can't feel any air movement. I checked around the booster, MC, and vacuum hose. I used a piece of empty tubing up to my ear to see if I could pinpoint where the sound was coming from too. It seems loudest inside the booster, under the MC.

[Ninjaneer11]

5 minutes ago, Zed Head said:

It's pretty well described around the internet that master cylinders can fail while bleeding them.  The ends of the bores never get used during normal fully-bled operation so they tend to corrode.  If you push the pedal through a full stroke you push the seal down to the corroded area.

If the pedal sinks with your foot pressure on it that's a sign of leaking seals.  

The other possibility for never getting a high hard pedal is that you accidentally switched the calipers when you had them off and you put the bleed screws on the bottom.  Not uncommon.

Thanks Zed Head. The calipers are mounted with the bleed screws up. The setup I have makes it impossible to mount them on the wrong side since the hardline from the caliper is fixed to the strut tube. I'll try bleeding the MC again tonight and if that doesn't work I'll start by just replacing the MC since they are readily available. If that doesn't fix it, I'll do it all again after getting the booster rebuilt. I'm going to be so good at bleeding brakes when this is done. 

[Terrapin Z]

Keep tabs on the reaction disc if you do pull things apart. Beats having to go back in when you reassemble with it dislodged. 

[Seppi72]

Power Brake Exchange lists two different prices for '70-'74 Z cars along with two different core charges.  I called and the more expensive one ($194) included the master cylinder.  But the fellow in San Jose told me they don't do MCs anymore so that leaves just the rebuilding your booster for $109 plus a $50 core charge.  There is nothing in inventory so you need to send in yours - I would assume the core charge is then waived - and wait for them to get the work done.

[Seppi72]

I just bought a new Beck Arnley 072-2694 Brake Master Cylinder on Amazon for $77 and free shipping.

[Ninjaneer11]

28 minutes ago, Seppi72 said:

Power Brake Exchange lists two different prices for '70-'74 Z cars along with two different core charges.  I called and the more expensive one ($194) included the master cylinder.  But the fellow in San Jose told me they don't do MCs anymore so that leaves just the rebuilding your booster for $109 plus a $50 core charge.  There is nothing in inventory so you need to send in yours - I would assume the core charge is then waived - and wait for them to get the work done.

 

26 minutes ago, Seppi72 said:

I just bought a new Beck Arnley 072-2694 Brake Master Cylinder on Amazon for $77 and free shipping.

Thanks for the info Seppi72! I have a zx master cylinder which works well with the bigger brakes. I ordered a replacement that I will replace this weekend. If that doesn't solve my issue I'll be sending off the booster to get rebuilt. 

[Zed Head]

On 10/4/2021 at 5:52 AM, Ninjaneer11 said:

I hear an air "breathing" noise when pumping the petal

There's a rubber boot around the rod that passes in to the booster.  And there's a valve in the booster that releases the vacuum when you move the pedal.  Not sure that the noise is a sign of a bad booster.  Might just be air moving through the valve and/or other orifices.

There are simple ways to check booster function beyond listening for engine speed changes, although the engine idle speed change is a good one.  If your idle speed doesn't change when you use the brakes your booster is probably fine.

Here's one of many booster check procedures out there.

https://www.knowyourparts.com/technical-resources/brakes-and-brake-components/vacuum-booster-diagnostics/