Cause of power brake failure?

[Peter C]

I have been experienceing problems with the power brakes on my truck. I've read through this thread and found that I did indeed have a bad vacuum pump and replaced it. But even now with a new pump, I have a hard pedal and not much braking ability. It's not completely gone, but mostly. Also (here's the part I haven't been able to figure out). When I have the brake pedal barely depressed there is a hissing noise (like a vacuum leak) that sounds like it comes from somewhere down near the pedal. As I press the pedal harder the noise gets quieter, until it goes away completely when I'm on it real hard. Any ideas about troubleshooting this? I'm assuming it's a vacuum leak somewhere (due to the noise, but I haven't had experience with anything like this before and haven't seen any other posts about it at all.

Thanks!

Peter

[Dave Haven]

Sounds like your brake booster is bad.

[Peter C]

What is involved in replacing the brake booster?

[tim94f250]

Not much. Remove the vacuum hose and check valve. Then remove the two 3/8 nuts holding the master to the booster. Pull master off booster and support with something. Under the dash, remove the cotter pin and slide all the stuff off the brake pedal arm. Remove the four retaining nuts and remove booster. That's it. Tim

[Mr_Roboto]

Yep, the booster is bad. The sound that you hear is the vacuum leaking straight through the booster diaphram.

Make sure you check the master cylinder. What will happen is the master cylinder will leak fluid into the booster, and the fluid will eat through the rubber diaphram. If the master cylinder is bad, it will do the same thing to your new booster.

[Peter C]

Thanks for the help! Sounds like a fun weekend I have ahead of me!

To clarify, if the master cylinder is bad the way you're talking about will there be brake fluid on/in the brake booster?

[tim94f250]

Yes. That is what he is saying. When you pull the master away from the booster, check the master for leakage. If you decide to change the master also, don't forget to bench bleed it. Tim

[Peter C]

Thanks for the clarification... now, if I get to that point and find that I do need to replace the master cylinder, do you guys recommend upgrading to the larger bore size cylinder? For those of you who have done it, what affect does it have on braking?

Thanks!
Peter

[cdnsarguy]

If you do change the booster, DO NOT remove the master cylinder push rod from the new booster or the rubber reaction disc will dislodge and booster will be damaged.

[Peter C]

Thanks for the warning!