77 GMC brake issue

[cam_114]

Two questions I'm seeking answers for..
1: part code for brakes "jb3" or "jb5"
I've looked all over the truck and internet, but no answers. I have available pics if they help.
The parts I may need are master cylinder and vacumn booster.

2: Brake light started coming on about a month ago, it would come on while sitting at a redlight. To disengage the light I'd shift to neutral, push the brakes..about half way or three quarters down, a sort of clunk noise would happen and light would disengage...
Then one day it just stopped.
So on the 16th I had both front and right rubber hoses at a local shop. They didn't bleed the brakes good at all. So I did them again.
Now the brake light started coming on again, this time its very hard to stop the truck,a lot of pressure is required, and I'm thinking only the rear brakes are engaging because of the recent snowfall and sliding to the right and zero traction braking.
I've bled them again and still same issue, and zero air bubbles.
I've tryed to diagnois the issue, I've started the truck and pushed the brakes, there is a hissing noise coming from booster.
Another thing to note, it appears fluid has, or did leak where the master bolts onto the booster, though I've never seen the master lose any fluid. Once when I first bought it the rear was low so I topped, no low fluid since.
I don't have a vacumn tester available to test it. I've visually checked the line, it seems good.

So any more tests to narrow the problem would be great...
I'm thinking its likely the booster, and or mc or possibly faulty rubber brake hoses...
Pics can be available...

Truck is c15 - 350 V8 front disc brakes, rear drum brakes.
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