I bought an old 87 f150 that sat for a few years and one of the brakes had a warp in it, making for a terrible shake whenever the brakes we're applied. I installed new loaded calipers and rotors today, and now the system won't hold pressure in the pedal. We bled the whole thing, but it won't hold. If the engine is off it will let me pump it up to get pressure, but when I let go its gone within seconds. When I start the engine, it won't hold pressure at all and I can't get it to pump pressure either. The pedal is very slow to come back up. There's a hissing that's been coming from pedal by the firewall that didn't affect anything before I did the brakes. What do I do??

 

The hissing could be a vacuum leak at the brake booster. Here is something to consider;
Through use sediment builds up in the master cylinder bore because the pedal only goes down so far with each application. On doing any brake work where you go to manually bleed the brake system you push the pedal all the way down to build up pressure and that sediment now cuts into the rubber cups and causes them to leak pressure . Now the master cyl has to be replaced. Look at it this way , its an investment in your safety because now you know at least the front will operate as designed. It takes a lot of brake fluid to fill the system and purge it of air so, get extra brake fluid you will need it. Don't forget to bench bleed the master cyl before installing it.

 

Thank you for your help! I'm not getting any leaking from the master so I was reluctant to go replacing it, but I always end thinking that it can't hurt any. How do you bench bleed it?

 

When you purchase a new master cylinder it will usually come with directions on how to bench bleed it. If you purchased a new unit and it did not come with bench bleed instructions and tubing , will try to explain the process which is not difficult at all.

 

I ended up going to the junk yard and getting another one out of an old bronco. I'm still not getting pressure, I'm not sure what the problem is at this point, time to call on the reinforcements that are my mechanical friends.

 

We, in the trade, NEVER use a junkyard master cyl. It is probably in worse condition than your original one. Even some rebuilt ones are defective.. My suggestion to you would be to get a new one , bench bleed it , then bleed the rest of the system. This will possibly be difficult with a vehicle that has not had the bleeder screws opened for a long time, but it has to be done to get the job done right. When you need that brake system to work , it has to work and work right !

Brake fluid has the tendency to absorb moisture. It is alcohol based. Just sitting in the vehicle it absorbs water and ruins the smoothness of the bore of the master cyl. You are taking a big gamble with a junkyard part there.

 

I got a new master cylinder ($20 isn't going to ruin me) so I'll put that in today. A friend will be coming over to help me as well who's a pretty savy guy mechanically. I'll let you know how it goes. Again, thank you for all your help! I've never done brakes that ended up being such a pain in the ***.

 

Just think if you didn't take care of the brakes that pain could have affected more than the part it has. It's all a learning experience. We have all been there at some time.