Hey he's a question for the experienced ranchero owner .When I got my 72 GT ,The motor was stripped and ready to pull ,I can't remember if it had a bean can vacuum canister .Could that be why break pedal keeps dropping and causing my lights to come on.
The vacuum canister is really just a reserve and/or a "buffer" that can help a vacuum signal to be a little more steady. If your idle vacuum is below 17in hg vacuum assisted brakes become less effective(of course the lower the vacuum reading the less assist the booster will give), the main reason I went with hydroboost. You can change the pedal ratio as sort of a "Band-Aid"to help, but this could have consequences like brake booster pushrod angle/geometry issues or even brake light switch problems if the change is extreme. If not wanting a HB install, manual brakes are another option.
The can was to provide vacuum to the A/C controls during acceleration, but I can't remember if heater-only controls in the '72-'79 used vacuum also, or just cabling, so not having one would not affect your brakes.
New master cylinder new booster new wheel cylinder s, calipers, lines ,I bleed to large bottles of fluid through system. No bubbles
I pulled master to check new adjustible rod to old one and when I pulled rod it had plenty of suction and was adjusted properly
I forgot to mention, if the dash "BRAKE" light is illuminated the rod/plunger within the proportioning valve may be off center, which usually means a leak and/or air is present on whichever side the plunger is off center. Another condition that could cause this but sometimes gives no visual sign of a fluid leak is a master cylinder that internally bypasses. Let's say the MC has a faulty O-ring seal on the piston that sends fluid pressure to the front brakes, the internal pressure will bypass (fluid goes back into the reservoir) and very little pressure will go to the front brakes whereas the rear brakes will receive full fluid pressure causing the prop valve piston to be forced off center toward the front thus causing the BRAKE light in the dash to illuminate. Just a thought.
What aquartlow just said is right on target. If the pedal is moving down with the same amount of steady pressure, most likely the master is defective. I have had instances where if you had a lot of pressure on the pedal it would hold position, but if I had slight to medium pressure the pedal would sink ever so slowly to the floor. It's sometimes a real chore to get the proportioning valve re-centered, but I don't know if the light is on constantly, or once in a while. The lack of a vacuum reservoir usually will result in a very hard pedal AFTER the initial "hit" on the pedal.
If the pedal is low when you first step on it, then improves, the problem is that the rear brakes are out of adjustment. If the pedal pumps up, gets higher with every push, you have air in the system. If the pedal slowly drops while under pressure, you have a bad master cylinder. Did you bench bleed the new MC before installation?
Cool. The brakes r fine. Pedal just falls enough to turn brake lights on .When I wake up in morning they r on if I pull pedal up slightly brake lights go out
Recheck the rears for adjustment then adjust switch for stop lights. We all were thinking the dash brake lite. And pedal fade. I have installed a pedal spring in the past mainly on manual cars that would light up the brake lights when pushing in clutch.