Brake Pedal Goes to floor suddenly out of the blue

[USAFSS-ColdWarrior]

Somebody please correct me if I am wrong but I don't understand how these vent caps are acceptable to use on the master cylinder. It is my understanding that they have a small disk that is held closed on the seat by a small, low tension spring. As the gear oil in the axle, transmission or transfer case, that they where designed for, heats and expands the pressure builds and moves the disk off the seat and relieves the pressure. Isn't this just backwards from the needs of the master cylinder. Lets say the the brake system has no leaks to deal with and just normal wear of the brake shoes is involved. As the shoes wear the wheel cylinder pistons will travel outward further and require more brake fluid to fill the space in the wheel cylinder. As more fluid is required by the wheel cylinders the level of the brake fluid in the master cylinder will decrease. If no allowance is made to fill the void left by the lower fluid level in the master cylinder a vacuum will form and brake fluid will cease recharging the master cylinder piston for the next application. So if we install a valve that is intended to relieve pressure, not vacuum, isn't this just setting up for a brake problem.

Your description is as correct as this former design engineer can surmise....

Use of these vents on a Master Cylinder is a Bass-Ackwards application and will more than likely cause more problems than curing any at all.

 

[Carl_in_NH]

These vents are indeed the wrong flavor for the application; the original MC vent was different from the other vents on the truck for a reason. Thanks for pointing that out - I didn't consider that when looking at the photo of the setup.

 

[JCKnife]

So those that don't have the vent cap have what instead?

 

[DieselBob]

So those that don't have the vent cap have what instead?

A vent line that connects to a "T" . No. 1 in the image.

 

[Chieftawny]

Sounds like a good reason to install a remote reservoir. Good eye Bob.

 

[Heavysteven]

Dito with diesel bob. A spring vent was incorrectly installed on the master cylinder. Why probally because it's easier to remove with that mod.

A remote resovior would be nice.

 

[rlwm211]

I leave the compression nut on the small copper tubing on my master cylinder slightly loose and it swivels when I want to open the master to check the fluid, and conveniently hangs there on the tube from the T when I am filling the master, or any other checks or maintainence. I do not plan on fording any streams as deep as my master is high in my truck so I do not worry about any leak into the master.

RL

 

[redjordan]

well I bled the entire system and have brakes again, but no brake lights. Took the truck for a drive in the front pasture for a good 20 minutes, all seems well. Going to let it sit till next weekend and repeat the pasture test and see if shes still got brakes.

How the **** did an air bubble get in there? No fluid loss, and no pedal always scares me. I should have gotten a 5 ton, these brakes are sooo funky.

 

[doghead]

The 5 ton systems are just like the 2.5 ton trucks, just bigger.

 

[dabtl]

Every time I see 'no lights' I come back to cranetruck's advice to check the grounds and connections. I do it at least once a month now.

Last week I noticed my right rear brake light was not working. I checked the bulb. Good to go. I got out the ohm meter and began at the light connection. Bingo! No need for the ohm meter. Loose connection. Lights work like a charm now.

You might solve the brake light problem just as easily.

 

[redjordan]

The 5 ton systems are just like the 2.5 ton trucks, just bigger.

I thought the 5 tons had real air brakes.

 

[clinto]

I thought the 5 tons had real air brakes.

M939 series are air brakes, M809 series are air assisted hydraulic, just like the deuce.

 

[redjordan]

Good to know. thanks!

 

[Ferroequinologist]

I highly recommend the remote reservoir-
http://www.steelsoldiers.com/deuce-...1-installed-remote-brake-reservoir-today.html