89 F150 Inilne6, 2wd 5spd, ABS only in the back

If you haven't used the brakes in a little while, because you're on the highway or something, when you first press them they go all the way to the floor, then the 2nd time you press them they have "pressure" and they work, you have to pump them once.

What is this caused by? I assumed it was a bad master cylinder and was going to buy one, since Napa has them for only 38$ remanufactured, but someone said it could be the Vacuum reservoir, or the diaphram inside the vacuum reservoir.

What does the diaphragm inside the reservoir do?

 

I am not a brake expert by any stretch but I'd be thinking master cylinder as well. The hydraulic system is closed and the pedal shouldn't go to the floor if everything is right with the plumbing. Even with a total failure of the power assist, you should still have normal brake function, you would just need a lot more weight on the pedal. That is how I understand it at least.....

 

I've got the same truck. I've noticed my pedal going soft (not to the floor) as well. If I pump twice it is solid.

You definitely have air in your system. Could be caused by M.Cylinder, booster leaks, wheel cylinders/caliper seals, lines (soft/hard).

My system has pretty old brake fluid in it. I'm going to bleed and flush it then see how it runs.


I'd recommend bleeding, replacing the fluid first.

Then - if not luck - replace MC then test and/or replace component by component until you are satisfied.

These are 21 year old trucks. Yours and mine probably have the original brake fluid in them.

good luck.


/Ricache