I recently noticed this after a recent job from the local dealership.

My brake pedal slowly goes to the floor and when I release it, the idle of the truck goes up and then settles back down. Do you think there is some sort of vacuum leak around? The truck does have a slightly shaky idle, rich smelling exhaust, and poorer gas mileage after the last job (they replaced a cracked exhaust manifold and an oil pan gasket).

 

Going all the way to the floor is not good. Does it do that when the truck is NOT running? If not, I'd guess you have a booster and/or vacuum problem.

If it fades all the way to the floor when its Not running you have a hydrolic problem-break fluid leak. Which means soon you will have no brakes

 

If it was a booster problem/vacuum problem the pedal would be high and hard, a pedal that slowly sinks to the floor would be a hydraulic leak or a bad master cylinder (internal leak). If it had a vacuum leak the exhaust wouldn't smell rich because a vacuum leak makes you run lean.

With the truck off pump up the brake pedal then hold steady pressure on it, if you can't pump it up or it drops when you hold pressure on it then you have a leak or a bad master. Now with pressure on the brake pedal start the truck, if the pedal drops some then the booster is good. I would take it back to the dealer, they probably jacked up the engine to replace the pan gasket, maybe they tweaked a line or something something? If it wasn't there fault they might want to charge you some diag time but at least you will know if the brakes are safe.

-Jon

 

This truck is a speed density/MAP and not mass air flow; so I do believe that fuel/air ratios can be affected by vacuum leaks. The higher the manifold pressure (lower vacuum), the greater the load, and therefore, more fuel must be added.

Nonetheless, with the truck off, I can pump the brakes to release any vacuum in the booster, and when I hold it steady, it slowly sinks. Once I start the truck, the pedal goes the rest of the way to the floor. Of course the booster works, but I need to know how to find out if there is a vacuum leak with the booster, because when I release the pedal, the idle goes up and then settles down. What happens to the idle of your truck when you release the pedal after holding it awhile? Does it change or hold steady???

 

Theres one easy way to find out if it's the booster. Clamp off the vacuum hose to the booster, or disconnect it and plug it and see if the idle still goes up and down.

 

If the pedal won't hold with the engine off, you've got a serious problem. Check brakefluid level immediately, you could be losing it, quite possibly into the vacuum booster.

 

I disconnected the booster and plugged the line, but no change to the idle (still a little shaky and slight hesitatation on quick throttle). Vacuum at idle stayed between 19" and 20", but went down to 0" on quick throttle instead of around 2".

Also, I applied about 10" of vacuum to the booster and lost about 2" over 10 minutes- not sure if there is a slight hole in the diaphragm, but I guess it is possible.

Fluid in the master has not changed (as far as I can tell), so there could be a little air in the line- I will chase that later.

Bottom line is the idle changes when I release the brake and was wondering if anybody else's does?

 

Every vehicle I have ever been in that has power brakes makes slight differences to the idle when the brakes are applied and released, but not enough to cause a problem.

If you have a vacuum leak in the booster, even a little one, the pedal will become hard. The bigger the leak, the harder the pedal. It's kinda like power steering. leaking fluid will make the wheel harder to turn.

If the pedal fades to the floor, yet the fluid level doesn't change, you either have an internally ruptured hose between the frame and a caliper, or the seals inside the master are worn, and the master needs to be replaced.

Take care,
~Chris

 

Thanks, Chris. I will focus on the MC- I know there is no air in the brake lines. Plus, I am going to try and chase a vacuum leak somewhere else.