I was hauling my 5th wheel horse trailer this weekend (with electric trailer brakes) and after dropping one horse off and starting out of the driveway again I hit the brakes at all of 15mph and the pedal went all the way to the floor with no resistance..I was stopped by my trailer brake. I pupped the brakes and then tried again and the brakes were normal. I have been driving it for a few days now and have not had any problem. I just want to have it fixed before hauling my horses again.

What would cause this, faulty vac pump line....air in brake lines? There were no leaks/ brake lines in tact the only thing I could find of the rims where hot...but the pads are only 2 years old and seem to be in good shape. Its a 99 F150 with only 79K miles on it.

Thank you for any input!

 

Could be a vacume problem. Inspect the soft lines to the wheels at all four corners. these can become soft/weak and collapse during heavy use, causing the symptoms described.

 

the hoses can have internal problems. you were hauling alot of weight so.. those brakes were working hard. the pedal to the floor and then normal again sounds like a master cylinder problem. those dont go bad often though.

 

If there was an issue with a hose you would be losing brake fluid and the pedal would still be soft.

What is more likely is that you over-heated the brakes and the fluid boiled. This led to air bubbles in the calipers and a spongy brake condition. As the brakes cooled more and you pumped them the air was re-absorbed into the fluid and the brakes feel fine.

BTW - this air re-absortion of air is normal (although you may want to bleed to brakes to reduce moisture in the fluid which can lead to boiling the fluid at lower temps). When the OEMs fill the brake system they suck a vacuum on the entire system and then inject the brake fluid at 150 PSI. I've watched brand new brake fluid "boil" at room temperature during this process.

 

I suspect the master cylinder had a brain fart. More specifically, I suspect the piston seals temporarily failed to force fluid to the wheel cylinders/calipers. Rare to see a one-time thing like that, but I've had it happen to me once before. As a precaution, you could replace the master, bleed the system. I've also seen replacement master cylinders faulty and do the exact thing you mention. The one I had went within a week of replacement; next, no problem.

Don't think the air-in-the-fluid getting reabsorbed flies though. To lose the pedal and have it come right back again tells me that air won't reabsorb that fast, if it is in fact even possible.

 

not sure if it would "fly" in canada, but down here in the US it does.
I am a master cylinder engineer - I have run tests in clear models (really thick lexan) and personally seen the results. I even have high-speed video showing the fluid boil and then the air reabsorbs and it is a good bleed, but I do not believe I can post it.

The theory on rolling a seal is possible, but chances are you would duplicate the failure. With the driver's description of pulling a trailer and having hot brakes, I think boiling the brake fluid is the most likely cause. If the problem occurs again when the brakes are not really hot, I would change out the MC. Otherwise I think it would be a waste of money.

I would still recommend bleeding the brakes to try and reduce the moisture content in the fluid. New fluid from a sealed container should have less moisture than what is in your system.