[ImperialCrown]

Do you still have the old parts to compare? Does the new booster have an adjustable pushrod and did instructions come with it? Is the pushrod there? We are assuming that these are the correct booster and m/cyl for the car.
The hiss is vacuum release from the booster vent valve letting air into the atmospheric-side of the booster chamber. A light hiss may be heard as you depress the pedal. A loud hiss may be a defective (torn diaphragm?) booster and you are hearing a direct manifold vacuum leak.

 

[valiant67]

The master cylinder replacement should not have caused the noise problem - I think it may have been there all along. If the master cylinder leaked fluid it may have also leaked into the vacuum booster where it can deteriorate the diaphragm inside. Or the booster may have rusted.

 

[tylerwrage1]

It was not hissing before. I now have no brake pressure. I didn't get a pushrod with the part, but I noticed that the rod where you connect the cylinder to the booster was wiggling around (I think that's the "push rod" are referring to), is it supposed to??? If I need to replace the booster, I will be disappointed.

 

[valiant67]

Hissing has to be a vacuum leak, the master cylinder doesn't work off vacuum. Only the booster does.

 

[ImperialCrown]

Is the hissing heard under the dash or under the hood? There may be a missing rubber o-ring to seal where the m/cyl mounts into the booster hole.

 

[tylerwrage1]

Okay, it WAS the booster, got it replaced (that was a load of fun). I went to bleed the brakes, the back bleeder screws broke off. After further review, they needed redone. After new hardware, shoes, cylinders, and turned drums, I thought I would be good. Got the new booster and master cyl. on, bled the brakes brakes, now they are braking, but it took me like... 6 seconds to stop when going only 40, so still not functioning good enough to be driven. Any ideas, PLEASE?!? I have all the vacuum lines hooked up, etc, they just aren't stopping!!! I am sick of it just sitting there, any ideas would be great!

 

[Bob Lincoln]

Does the pedal travel far? Is it firm or squishy? What procedure did you use to bleed the master cylinder and the brakes? You may still have air trapped.

 

[tylerwrage1]

The first 1/2 inch or so, I have nothing, which, in reality, may mean nothing. We just air-bled them. So, as of right now, we have it down to either: still air in the line - gonna power-bleed them tomorrow, collapsed line, going to attach clear line tomorrow also to check flow if fluid. It may also be that the master cyl. we put in was bad. It had been in the trunk for two years according to the guy who had it last, but it was in perfect shape. I have asked some people, and they say that the (cups?) in the master cyl. may be bad. I think the push rod between the booster and the master cyl. is the correct length, but that may be a potential problem as well. The second half of the brakes are pretty average, but they don't stop even enough to drive it safely. Thank you for your help, by the way.

 

[Bob Lincoln]

So, let's go over detail. Did you bench-bleed the master cylinder before hooking it up? That is mandatory. There are two ways to do it. One is to install plugs in the line outlets and, with covers off and cylinder full, hold the master cylinder in a vise and push the piston slowly, over and over, until the bubbles stop and travel is about 1/8" when pushing by hand. The other method is to install lines at the outputs which loop back into the reservoirs and are submerged. Then you slowly pump the cylinder until the bubbles stop. Either way, then you install the lines from the car and bleed the 4 wheels.

When you say 'air-bleed', what you should be doing is working from furthest wheel to nearest (so, RR, LR, RF, LF). At each wheel, you connect a hose from bleeder screw to a jar of clean fluid and submerge that end of the hose. Have one person step on the brake pedal and hold it down. Then open the bleeder screw until air bubbles shoot out, and close it, and then let up on the brake pedal. Repeat until no more bubbles come out. Do each wheel this way, refilling the master cylinder constantly so it doesn't get low enough to trap air.

Is this how you bled it? It should be effective if done right.