2nd Gen Non-Engine/Transmission Intermitten low brake pedal

[Gordon Maney]

In the last two days I have had a few instances of a low brake pedal. That this is possible flies in the face of all that I think I know about brakes. It is noticed at a stop or low speed. As soon as I am moving at any speed, the pedal is high.

Thoughts on this?

 

[RStankevitz]

sounds like low vacuam for some reason mine does this if i have the heater on the floor there is a leak in the selector

 

[Gordon Maney]

I checked the master cylinder level and it is full, and there is no sign of leakage anywhere. I am wondering if it is an ABS issue. I have had instances in vehicles with ABS that when you hit bumps in a certain way the pedal would go down.

 

[JLandry]

I believe our brakes (on diesel trucks) are boosted hydraulically by the power steering pump (not vacuum).



Have you checked the power steering fluid level?



I assume you've checked the brake fluid level... yes?



Best regards,



John L.

 

[Gordon Maney]

I have checked the brake fluid, but no the steering fluid. I will certainly do so. The thing of it is, the brakes are fine 99% of the time. Only occasionally will they pedal go low, otherwise they are entirely fine.

 

[JLandry]

I have checked the brake fluid, but no the steering fluid. I will certainly do so. The thing of it is, the brakes are fine 99% of the time. Only occasionally will they pedal go low, otherwise they are entirely fine.

Gordon,



Some more thoughts...



If you have drum brakes in the rear, then you might try adjusting them.



Now assuming the power steering fluid level is normal and isn't the cause...



When it happens again, try quickly pumping the brake pedal and see if that immediately restores it to normal height. If it does, there's a good possibility the master cylinder has an internal leak where the brake fluid is by-passing the cup seals. If this is the situation, then you might try flushing out the brake system with fresh fluid to see if that clear out any contaminants.



If that's not the case (you can't pump up the pedal), then I suppose it's possible you just have a power steering pump that's getting worn out. One way you might try to check this, is when the pedal is low (when stopped) try rapidly moving the steering wheel back and forth to see if it feels like you have less power... like the power steering pump isn't putting out it's normal pressure.



Best regards,



John L.

 

[Gordon Maney]

I have disc brakes on the rear.

I admit to not having experience repairing hydraulic boost systems, only vacuum. Having said that, I would think that a properly working boost system would not cause the pedal to rise. A vacuum booster does not affect pedal height.

Is there something different about hydraulic boost?

 

[TboneMan]

When was the last time you changed the brake fluid?



I had some great advice (from this site) a few years ago when I was experiencing a similar problem. Brake fluid can absorbed water. This can cause problems over time. Changing the fluid solved the problem.

 

[WilsonF]

It could be the power steering pump, at lower RPMs it is turning slower, less pressure and less volume. I read on this site or DTR, I forget which, about the pump filter getting clogged and the procedures to clean it, the thread also had steps to modify the pump to put out more. I think he, the guy that started it, had the same problem, notice I didn't use the word issue,that you describe.
Floyd

 

[WestTN]

Intermittant low pedal as you have described is likely caused by the master cylinder failing. It can and will eventually just fall to the floor and leave you with no brakes. The rubber piston cup or the cylinder wall is wearing out.

 

[Gordon Maney]

When was the last time you changed the brake fluid?

I had some great advice (from this site) a few years ago when I was experiencing a similar problem. Brake fluid can absorbed water. This can cause problems over time. Changing the fluid solved the problem.

I have never flushed the system on this truck. That is a good idea no matter what.