69 avanti II brake issue

[Avantir4]

Drove the Avanti to Duane Millers funeral, car has been running great. during the funeral procession, when stepping on the brake pedal, brakes felt spongy 

then heard what sounded like firecrackers going off, only when stepping on the brake.  When getting home to put the car in the garage, stepped on the brakes they went to the floor,and firecrackers going off.

Ordered a new dual master cylinder since my car has turner disc brakes, bench bled the master cylinder, after bleeding the brakes, took the car out, brakes go the floor.

Purchased new calipers, bled the brakes, brakes go to floor. took master cylinder off bench bled again, , bleed brakes, brakes go to the floor

Ordered another master cylinder and going to try this one.

Could i have a booster problem ? 

Frustrated 

[Gunslinger]

You might want to replace the rubber brake hoses…especially if you don’t know how old they are.  They can look ok externally but internally go bad.  
 

By any chance…did the brake fluid get replaced by silicone DOT 5…or already had DOT 5 replaced by DOT 3 or have them mixed?  They’re not compatible and will make the fluid turn into a milky looking mess that will give a very spongy pedal if not fail altogether.  If that occurred a complete flushing of the brake system is required along with new hoses and seals.  
 

DOT 5 also requires a thorough bleeding of air.  Any bubbles are much smaller and can be a bear to get rid of.

[mfg]

Any brake fluid under the car?… kind of sounds like a rusted steel brake line blew.

[Avantir4]

no leaks in the system , no brake fluid on the floor , and the brake fluid i used was WEAREVER BRAKE FLUID DOT 3, and when bleeding the brakes , pumped the pedal at least 15 times per time, starting with the right rear , then left rear, right front and left front 

 

[Zedman]

Avantir4-

I remember having a devil of a time getting "flow" having opened up the Brake System on m y 321i BMW convertible years ago.  I pumped and bled and even tried gravity flow to get it to go, but ended up trailering it off to a brake specialist. They told me attaining flow was the problem.  I would wonder whether one of those little handpump self  bleeder kits is what we need here.... the kind that push fliud back UP the system from the Brake Bleeder Valve. I'm putting one of these on my 'to-d'o list once my physical health improvesand I get back to wrenching.

 

[Avantir4]

Definitely worth a try , The Avanti has an appointment over at the brake shop Thursday , we will see what happens, thanks for the information