I have posted about this in other threads and before anyone recommends it, know that I am planning on upgrading to the Hydroboost system next year once I can afford it. In the meantime, I feel like I am going through vacuum pumps at a ridiculous rate. The last time I thought the pump was going out, I replaced the pump and the vacuum booster on the brakes (on a recommendation from someone on this page). That was about 8 months ago.

Fast forward to this past Sunday when we are driving home from a local camping trip towing our 12,000 fifth wheel. I had to brake suddenly and my foot went nearly all the way to the floor. I had to reach down and use the trailer brake controller to stop us. From then on, I was driving very cautiously to ensure we made it home safely. The brakes didn't have any issues the rest of the way until I got to the storage lot to drop off the trailer. Backing up, the brake pedal went hard as concrete and I had to stand on it with both feet to get the rig to stop. Driving the truck home from storage without the trailer, the brakes would go in and out from a hard as a rock pedal to going nearly to the floor with little response.

I checked all the vacuum lines and everything seems to be fine with no leaks. The vacuum pump looks good but I know that doesn't really mean anything. I ordered a new pump and we are leaving next Tuesday on another camping trip. I am a bit anxious as the final few miles of the road leading to the campground is narrow and steep...not the place you want to have a brake issue.

Could I be missing something? I am going to check the vacuum on the pump before I replace it but is there something else that could be causing this issue?

 

There is a T that goes to your heat/AC controls. once that leaks, brakes can be a bit weird.
look under hood near heater box for the vac lines and follow them through the loom looking for breaks


when it went to floor, did it stop you at all? if it did not then get a new MC and gap it to booster .020
if it did stop you but keep going to floor, then you might not have a zero loss booster and a quick pump brings it back, if not then i would replace booster with zero loss and get a new MC

another thing overlooked at times is RABS module, it needs to be bled and they do fail over the years and can cause similar to what you are seeing

 

I don't know what to say except cheap China made parts. The brake peddle going to the floor says master cylinder to me.

 

I don't know that the MC is the issue since it seems to go away every time I replace the vacuum pump.I did find one on Amazon for $35 so probably worth a shot. I didn't try pumping the brakes but it did stop eventually. I did rely heavily on the trailer brakes but could tell the truck brakes were engaged although not fully. I will check all lines and connections but I didn't see any failure points. As far as the RABS, I am not getting any warning lights which are usually synonymous with the RABS going out. Could be the issue but highly unlikely.

I was thinking the same and will replace the MC when I do the conversion. It really seems to only happen when the pump goes out or I am towing. I did find the MC on Amazon for $35 so I may just replace it now. I need to check vacuum pressure to see if the pump has gone bad but my guess is yes especially with the hard pedal issue.

 

Dumb question but was your E brake engaged when the pedal went to the floor?

I say this because the other day I forgot to disengage the dam e brake and drove the truck like that thinking I had a heavy load (which I did 15k) but felt heavier than usual, drove like that for 40 miles when like you had to use the brakes suddenly only finding myself with pedal to the floor with no brakes, pulled over the side having the trailer brakes stop me, once I notice my stupidity I committed I let the brakes cool off the brakes worked good again, drum was glowing red and rim was hot that I spitted on it and it sizzled lol.

Might want to check if your brake shoes are working properly.

 

When my vac pump died, booster was fine and was rock hard...due to no assist. It also nearly destroyed my Fleetrunner.

 

Not a dumb question at all. I can assure you it was not engaged for two reasons: 1. My brake light would have been on and 2. the truck is nearly impossible to move with it on. I have it adjusted properly so that it will hold the truck and trailer when parked on a hill.

 

After I did my hydroboost upgrade I also went with a Super Duty electric vacuum pump. All it controls now is the dashboard vents and no more worries about mechanical vacuum pump bearing failures.