Losing Brake Pedal Pressure

So I finished the Airpack rebuild and the brakes do work.. Adjusted the rears so they are not dragging anymore. So now I slowly lose the pressure in the brake pedal if let's say I am going down a long hill. Even if I am sitting at a stop sign it slowly goes to the floor. The pedal itself springs back up when I let off the brake but the pressure behind it takes some time to come back up. Based on similar issues others have had my guess is that I now need to rebuild the Master Cylinder. Your thoughts please..

Thanks!!

Sounds like a leak somewhere. MC rebuild is a good idea. How are the wheel cylinders? Any leaks in the brake lines?

The wheel cylinders are a ? as I have not yet opened up the hubs to take a peek.. I assume they could also probably be due for a rebuild. The M/C rebuild kit is cheap so I think I will just go ahead with that and see how it goes. I do not see any fluid leaking out anywhere.. anymore that is.. I did have a bad brake hose in the front that I replaced along with a few lines that broke when I removed the airpack..

Did you bleed the system real well and use the right brake fluid?

I did bleed real well, used Dot 3 (which was in the truck already) and I plan on bleeding at least 1x per year.. Perhaps there is still some air caught in the line somewhere..

Are your loosing brake fluid? Does the level go down after several pedal pushes(and leak downs)?

Have you pulled the rubber dust boot off the master cyl, and looked for leakage?

No Fluid loss would indicate an internal leak in the system where the fluid is BYPASSING a seal.

If the air pack was bad, I can assure you the master is equally as bad. I am talking about sludge and corrosion as well as seals that do not hold pressure.

Look for leaks at your wheel cylinders which would show up dripping out of the drum area and onto the rims and the back of the tires.

Dot 3 is the WRONG brake fluid, You need dot 5

If someone put dot 3 in im sure it could screw up seals and other things. I think you might have to bleed your brake system completely and put in dot 5. dot 5 is a synthetic brake fluid and dot 3 is not

Digger09 said:

Dot 3 is the WRONG brake fluid, You need dot 5

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DOT3 will work if system was properly flushed before putting it in.

DOT 3 vs. DOT 5
Brake fluid -how to tell which kind you have DOT 3 or DOT 5

. dot 5 is a synthetic brake fluid and dot 3 is not

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It might be less confusing to say DOT 5 is silicone based (actually an organic type of silicone).

It is typically 95% Polydimethylsiloxane.


Other brake fluids are commonly called synthetic.

porkysplace said:

DOT3 will work if system was properly flushed before putting it in.

DOT 3 vs. DOT 5
Brake fluid -how to tell which kind you have DOT 3 or DOT 5

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I did not know that. Thanks for enlighting me to it. My bad.

Ok, now back to my issue, just got out from under the truck.. The moron who rebuilt the Airpack didn't tighten the nut enough for the brake sensor.. yeah that's me.. So half the issue fixed.. The pedal does come up rather quickly now so I can pump the brakes again. Now for the other half of the issue. If I press down on the pedal and hold it it slowly goes to the floor which would indicate a leak somewhere but I am not seeing anything dripping out. The M/C looks pretty dry from the outside especially around the boot. The M/C looks "Newish" compared to what the airpack looked like you know rusty vs. green. I would still think it's a possible internal issue in the M/C or a wheel cylinder but there are no leak spots on the wheels or rims. Hmmm..

Sure sounds like an internal leak in the master cylinder to me.

How rusty was the hydraulic bore of the air pack?

If it was bad, you need to attend to the Master Cylinder. CHances are better than pretty good that it will be as bad, or worse than the Airpack.

DW11, if the airpack and maybe the master cylinder are bad, the wheel cylinders are probably in the same condition. You really need to pull the wheels and hubs and check them. Wouldn't be a bad idea to inspect and repack the wheel bearings too.

Look at the rubber brake lines while you're at it. They are not that expensive to replace.

The airpack was pretty bad.. I posted my rebuild with lots and lots of pics.. here is the link
http://www.steelsoldiers.com/deuce/82753-my-airpack-rebuild-photos.html

I think I will do the M/C either way, the rebuild kit is cheap and after doing the airpack this should be a piece of cake.. Just have to see how many brake lines I break taking it out..

Thanks Guys!

DeucesWild11 said:

The airpack was pretty bad.. I posted my rebuild with lots and lots of pics.. here is the link
http://www.steelsoldiers.com/deuce/82753-my-airpack-rebuild-photos.html

I think I will do the M/C either way, the rebuild kit is cheap and after doing the airpack this should be a piece of cake.. Just have to see how many brake lines I break taking it out..

Thanks Guys!

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I'll bet none or one! If you break more than one, you had to try really hard.

Broke two lines taking out the Airpack, and one line replacing the front brake hose.. I figure between one and two replacing the M/C

Ray, thanks for the input.. I do plan on a tear down to inspect everything but don't have the time for that at the moment.. Wife's got me doing a Kitchen remodel...I don't drive the truck that often so I am not in a big rush..

Well, since there is only one tube attached to the MC, I was trying to be funny. I guess you could break the vent line too, that would be more than one.