I need to read the symptoms a couple more times to make full sense of this.
The air valve is working on the booster, and they replaced the tiny seal in the air valve piston (the one on the bottom side, at an angle)? These booster are not normally hard to rebuild, but it does sound like a booster problem.
The typical problem with MC rebuilds is that they don't properly port/unport the passage that draws new fluid in from the reservoir. But if you open a bleed screw, pump fluid through, the fluid in the reservoir drops?
I have done a ton of these systems, recently a 67 Super and a 66 GTV, but each brake rebuild can present its own fun diagnostic issues.
Andrew

 

Follow-Up Test- Isolate Front Brakes with Booster

Thanks for the fast response. Yes, all seals in the booster were replaced.

Test 3- Re-introduce front brakes to the system. Reservoir, M/C, booster and L/R front brakes only. Rears blocked at the firewall.

I bled the left and right front calipers and have a good pedal again. With the engine running, a get a short "hiss" from the booster when releasing the brake pedal. I assume this is the vacuum pressure being released.

I'm not confident the good pedal will last, as I've had a good pedal following a bleed on several occasions.

Booster Details- I'm willing to pull the booster and open it up. Is there a particular failure to look for? Thanks in advance.

 

Is the hiss inward or outward? A vacuum leak would be in the check valve at the manifold, hose, or chamber. An air leak would be at the air valve (the pentagonal thing). Does it hold vacuum after you shut off the engine? If not, could either a leak on either side of the system.

Can you plumb around the booster as a test? Pedal will take a lot more pressure, but if it holds a good pedal then, you know it's the booster.

This car doesn't have a proportioning valve does it in the rear line?

Andrew

 

Test 4- Vacuum Testing the Booster

Regarding "hiss"- The sound is very fast (less than a second) and I cannot locate where it's coming from.
Proportioning Valve- There is no proportioning valve in this system.

I removed the booster from the car.

-Confirmed diaphragm doesn't have any brake fluid in the chamber.
-Removed piston underneath the pentagon vacuum chamber. The piston and seal is clean and undamaged.

Vacuum Test- I capped the brake fluid inlet and applied vacuum to the brake fluid outlet using the MityVac. Then I sprayed soapy water around all joints and unions to try to locate any leaks. There was a very slow leak (less than one pound of vacuum per minute) which I'm guessing is negligible. No joints or unions showed signs of leaking in the soapy water.

I will try bypassing the booster altogether.

 

If there's air getting in, coz pumping-up becomes evident, surely there's fluid getting out.
If there's no evidence of fluid leaking into or around the booster, are you CERTAIN that all joints for your new lines and components, and bleed nipples are dry?
I've had and still have booster issues, but absent visible leaks, that's about vacuum on/off transition, with no affect on pumping up or not, bar the usual firming-up of the pedal as accumulated vacuum dissipates with the engine off.
I see no harm is testing booster by bypassing it, but absent visible leaks, I will be surprised if you'll find a change. Of course....I am often surprised.

 

I had a similar problem with a dual system with bonaldi boosters. I rebuilt one of the boosters twice. the second time I swapped the cylinder from another one, I have been told the brake light switch can leak air in with no fluid leaking out. it sound like you need to re do booster.
it drove me mad trying to find the problem and still i'm not sure what solved it.

cheers Ian

 

A hard line brake booster bypass was installed to confirm the rest of my braking system was functioning properly. The result was a very solid pedal that requires more pedal pressure to stop the car. This confirmed the booster was causing my issues.

I made a discovery on the single circuit brake servo issue. A Mini enthusiast explained to me that Bonaldi boosters were actually an upgrade to Minis from the 1960's. Some research unearthed a document from Moss Motors (British car parts specialist) describing the correct way to bleed a single line brake servo:

https://mossmotors.com/media/instructions/182-290_INST.pdf

The document states that the booster needs to be angled up at a 25-45-degree angle and rotated ~30-degrees in order to purge air out of the system. I unmounted the booster from it's mounting points to achieve the stated angles and then cracked the hydraulic fittings to purge any trapped air pockets. Achieving the stated angles is a bit of a challenge with the brake hard lines, but the procedure finally resulted in a great consistent brake pedal.

I'm still mystified how the brake boosters were installed and bled successfully at the factory, but I'll save that for others to figure out. Hope this helps others get their braking systems sorted out.