when i first got my 1984 el camino there was hissing in the brakes when i put my foot on them. i bled the brakes and he hissing still was there. i never had a problem wit the brakes until now. when i put my foot on the brakes i have to pump them in order for my car to stop. if i keep my foot on the brake long enough without pumping it, the peddle will loose pressure and go all the way to the floor. to fix this problem i bought a brand new master cylinder, that didn't work, and the car still doesn't want to stop. i go trough at least a bottle in a half of brake fluid in a day so there seems to be and obvious leak somewhere. someone told me that my proportioning valve may be going out, and i've heard that i might have a busted brake line. what do you guys think the problem is???
I'd make sure the master cylinder isn't defective first. You can have someone pump the brakes and see if they're pulling in air from somewhere with the reservoir cap off. It's definitely leaking somewhere.
i bought a brand new master cylinder and the hissing never stopped...in fact the brakes seemed to get worse even after i bled them. 
assuming you have power brakes:
the power brake booster diaphram may have a pinhole in it.it can cause the brake pedal to get hard;can cause the brake fluid to be pulled from the rear of the cyl past the seal and several other fun things.also inspect the vacuum check valve at the booster to see if it leaking vacuum and producing your noise.if so,inspect the rest of the system for leaks-you may have multiple problems.
the power brake booster diaphram may have a pinhole in it.it can cause the brake pedal to get hard;can cause the brake fluid to be pulled from the rear of the cyl past the seal and several other fun things.also inspect the vacuum check valve at the booster to see if it leaking vacuum and producing your noise.if so,inspect the rest of the system for leaks-you may have multiple problems.
Random thoughts and observations
Did you bench bleed the master cylinder before it went on? If not, you might end up removing it and doing that.
How much fluid did you push through the system? A full large brake fluid container worth yet? It's not unheard of to use 2 of the large cans chasing air in the lines.
It can be extremely hard to get air out of the rear lines, up to the point of requiring the car to have the rear jacked up to make the bubble move out.
Are there ANY wet spots on the brake lines, calipers, etc? Even the smallest leak will introduce air immediately.
Most GM power boosters of that era hiss at certain points, they're just noisy. If boosters go bad, the brakes get hard, too hard to effectively push.
So if they pump up, it isn't power booster, it's air in the hydraulics. You wouldn't be the first to replace a bad master cylinder, and have the brakes get worse due to incomplete bleeding.
Good Luck!
Did you bench bleed the master cylinder before it went on? If not, you might end up removing it and doing that.
How much fluid did you push through the system? A full large brake fluid container worth yet? It's not unheard of to use 2 of the large cans chasing air in the lines.
It can be extremely hard to get air out of the rear lines, up to the point of requiring the car to have the rear jacked up to make the bubble move out.
Are there ANY wet spots on the brake lines, calipers, etc? Even the smallest leak will introduce air immediately.
Most GM power boosters of that era hiss at certain points, they're just noisy. If boosters go bad, the brakes get hard, too hard to effectively push.
So if they pump up, it isn't power booster, it's air in the hydraulics. You wouldn't be the first to replace a bad master cylinder, and have the brakes get worse due to incomplete bleeding.
Good Luck!
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