86 F350 4x4

On my way home yesterday I noticed the brake pedal was going close to the floor but not all the way and I still had some braking power. Before I normally had a 3" drop on the pedal and maybe another inch before bottoming out. I got home and checked the fluid in the master cylinder. Both were half full so I filled them up. I tested the brakes out and still to the floor pedal. I checked the reservoir levels and the rear (larger) was still the same, the front reservoir was down to half again. I did a visual check of the lines and saw no leaks and saw no fluid running down the front tires. I replaced the front pads about 2k miles ago and replaced the front right caliper at the same time. Other than replacing pads and shoes, I know nothing about diagnosing brakes, what's it sound like?

 

Well, it is leaking out somewhere so it's just a matter of letting it sit on some cardboard with the whole front end over it. With the engine running press and hold the brakes for a minute and do that several times.

Being that the caliper was replaced recently, I'd guess there first. Check the nut with a flare nut wrench for tightness. Being as old as it is the lines could be rusted. If so, I'd replace all of them, otherwise you could have multiple failures like I did on my first 89 van. I lost the right front, and before I could drive even a mile, I blew out the rear line.

It could also be leaking between the master cylinder and the brake booster (that the reservoir sits on). There is not much fluid there being leaked, so you'll have to look close. The threaded connections to the master cylinder can be leaking also.

 

A leak that is not detectable and does not appear to be getting anything wet, may be leaking into the brake booster itself. The seal that seperats the booster from the master can get damaged and allow the booster topull vacuum from the master. If it is not too bad, it may not smoke out of the exhaust,but you should be able to smell the odor from the exhaust. It may not be too stong but it can be the only explaination for fluid loss if there are no drips, and the fittings are dry.

 

I finally got a chance to look around. The brake seems to work fine when first used but after that you can hear a hissing sound from under the hood as soon as the brake pedal passes the normal travel. When it stops raining I'll pop the hood and have someone press the pedal while I listen.

 

That hiss is probably a vacuum leak and you likely will need a new master cylinder.

 

How would I know if the power booster was going bad instead of the master cylinder?