brake pedal to BOTTOM OUT OR NOT

[ohnuts]

My M35A2 (77AMGENERAL) when left one or two nights will lose all air pressure. The Brake pedal after the system has bled down will drop to the floorboard when depressed with no resistance.

After the truck has started with almost no air showing on the tanks. The Pedal responds after one pump and is rock solid.

My M185A3 with empty air tanks before start-up has a firm brake pedal. With empty tanks the M185A3 is like a rock and feels just as good with 60 to 120 psi. Now I have never tried to move either truck with empty tanks. But one member said a brake pedal was more likely to be soft while there is air in the system.

So far I have fixed a couple bad airline connections. Replaced the regulator and fixed a leaking air pressure alarm switch. Flushed the brakes and filled the system with a pressurized tank. Felt great till left over night.

Is the brake pedal supposed to bottom out with no air pressure? If not (Master cylinder seems tight) then why does the M35A2 drop. The M185A3 hasn't dropped a bit in a week?

Will a bad slave or airpack drop the master cylinder and brake pedal? But drive great?

 

[m16ty]

The brake pedal shouldn't go to the floorboard with or without air pressure. Sounds like you my need to bleed the brakes.

 

[gimpyrobb]

At least check the fluid level in the master cyl.

 

[ohnuts]

Used a one gallon pressure tank to fill all the lines and slave cylinders with new dot 3/4 after blowing the system clean with twenty psi air. The Master cylinder was and is topped off. After air pressure comes up you can really stand on the brake pedal and there is no bleed down. Have the one inch slack on the pedal and almost no air leaks now.

 

[m16ty]

I blew a line on my truck and after I got it fixed and bled it did the same thing. I figured I didn't get all the air out. I just kept driving it and it finally went back to normal.

 

[Speddmon]

I'm no expert by any means, but the way I understand the deuce brakes is that they are hydraulic with an air assist, similar to but more complicated than, the vacuum assist in a regular car. If the pedal goes to the floor but comes back after the first pump, it sure sounds like air in the hydraulic side to me. Fluid will not compress but air will, the first pump compresses the air and then once it's compressed, the pedal feels normal again.

Please, someone correct me if i'm wrong.

 

[Jake0147]

The brake pedal should NOT fall to the floor with or without air supplied to the airpack.

The fact that they work tells me that the master cylinder, wheel cylinders, lines, and hoses do not have any (significant) leaks. It also tells me that there is not enough air trapped in there to take up a full stroke of brake fluid. Removing air from the airpack (removing assist) should make it harder to compress any trapped air, not easier, thus a higher pedal.

Since there's no leaks, and no excessive bypassing within the master cylinder, then to my knowledge that really only leaves the possibility of bypassing within the airpack...

 

[ohnuts]

Does the master cylinder only feed the airpack and the airpack feed the brakes? If that is true then I'm thinking it has to be some kind of bypassing in the airpack like Jake says.

 

[cranetruck]

This dwg courtesy Gringeltaube, shows you how the master cylinder will operate the wheel cylinders without any air pressure. Note piston "P", it has a valve within it, so that the fluid will go right thru it when there is no air pressure, this also lets you bleed the brakes without air pressure. The brakeline pressure will be about 400 psi without air and about 1,800 with.
Once bled, the brake pedal is hard at all times as stated above. If there is no loss of fluid in the MC, then there is no leak (like into the airpak via a faulty/stuck valve).

I would suggest that you bleed the brakes one more time, letting more fluid drain from each wheel cylinder. Look at the airpak in the area where the master cylinder line connects, there should be a bleeder valve there also IIRC, bleed this one first.

 

[JDToumanian]

In addition to making sure all components are in top-notch condition and that the system is properly bled, as recommended above, your brake pedal return spring is obviously missing or broken. It connects the clevis pin on the brake pedal bell-crank to a sheet metal tab with a hole in it sticking off the frame about 8" in front of the bell-crank. If this spring were present, your pedal could not possibly fall to the floor, even if the system had neither air nor brake fluid.

Regards,
Jon

 

[ohnuts]

Jdt

Sorry I was not clear the pedal can be pushed to the floor with no back pressure from the hydraulics. The pedal does sring back with the proper movement from the return spring.

Cranetruck told me of the bleeder on top of the airpac itself. I will try that now.

 

[pittdog]

bleed ... Or mst cyl problem .... Trust me ... 52-d... Ret... Usa

 

[Somemedic]

Ive been tracking down air leaks for the past week. I know my firetrucks can sit usually for days without being started and still hold pressure. My deuce will not hold air over 4hrs.

I located a device on the firewall which I believe is the governor. Fixed a leak there. The emergency gladhand leaked an was replaced. The airhorns had a slight leak which was repaired. The valve under the glovebox (whats it for anyway? Just another tapper for a blow gun?) got capped since I cant seem to find a similar replacement. And the front axle switch also got tightened.

It airs up a little more quickly but still leaks down after a few hours. Its more of a peeve that Im trying to come to terms with but still believe I may win this fight.

Whats strange is that when I first get in the truck to fire it up and take it for a spin the brakes do feel soft but after depressing the pedal a few times it feels much firmer. The brakes feel fine for the rest of the day, its only the first few blocks the brakes do this. Ive bled the wheel cyclinders several times and thought I paid attention to the TM's proceedure, but I dont remember bleeding the master cyclinder. I didnt notice a fitting...?

 

[ohnuts]

Somemedic you have the same disease I do and I haven't fixed it yet either. I'm going with Crane trucks idea when I get to crawling around. Also want to look at fabbing a remote tank for filling and storage.

 

[littlebob]

Ive been tracking down air leaks for the past week. I know my firetrucks can sit usually for days without being started and still hold pressure. My deuce will not hold air over 4hrs.

I located a device on the firewall which I believe is the governor. Fixed a leak there. The emergency gladhand leaked an was replaced. The airhorns had a slight leak which was repaired. The valve under the glovebox (whats it for anyway? Just another tapper for a blow gun?) got capped since I cant seem to find a similar replacement. And the front axle switch also got tightened.

It airs up a little more quickly but still leaks down after a few hours. Its more of a peeve that Im trying to come to terms with but still believe I may win this fight.

Whats strange is that when I first get in the truck to fire it up and take it for a spin the brakes do feel soft but after depressing the pedal a few times it feels much firmer. The brakes feel fine for the rest of the day, its only the first few blocks the brakes do this. Ive bled the wheel cyclinders several times and thought I paid attention to the TM's proceedure, but I dont remember bleeding the master cyclinder. I didnt notice a fitting...?

Can't remember if its on the airpack or M/C but there is one in that area that I need to bleed also. I bled it by myself with a vacuum bleeder and then did the rest of the wheel cylinders with my son( the better method without a pressure bleeder ) It's still a little spongy.