Brake problem

Yesterday the wife was driving Grugly on the highway and went to get on the off ramp and had no brakes. She said the pedal went half way and got a hard feel. Then no brakes, she stayed o n the highway and got off on the shoulder. Said she got it stopped with the parking brake and really smashing the brake pedal. Called a tow truck and when the driver put it on the rollback he told her it had no brakes. Got home last night and checked it over and no leaks anywhere. Also the brake fluid and powersterring level was good. I ran it up and down the driveway and it had brakes. Took it out on the road for a couple of miles and it had brakes. Stopped with no problems. She said last week that the pedal felt hard and when I drove it there was no problems then either. The next day she said it didn't feel hard. She has been driving this truck every day for awhile now, so I believe she would know the way the truck feels. Not a normal woman, she can handle trucks. Would this be the hydra boost going bad? Maybe the pedal or rod sticking? Bad master cylinder? She won't drive it till I get this figured out and I'm not sure what to do at this point. Don't want to just throw parts at and it not fix it? Any ideas?

When the brakes went out did she still have power steering?

She said yes. Also had no problems with steering last night.

I'd have to tell you to get a set of brake pressure gauges and see what it's got at each caliper..

usually the first sign of a hydroboost unit going bad is a pedal that's reeeeeeeeaaalllll slow to return..

Would the hydra boost go out randomly? Where would one get brake gauges? Never heard of them. What pressure should be at each wheel? Could this be a problem with the abs system? This is awhole new problem area with this hyrdra boost system to me. Need to get this fixed, because the wife wants her truck back on the road.

Any other thoughts? Really need to try to figure this out this weekend. Anyone have a idea how much a new hydra booster is? Or how hard they are too replace?

Personally, I think I would replace the Master cylinder. But be ready to replace the hydroboost also.