Master Cylinder Vent Line vs Breather

So the my chronic brake switch issues have made me a better mechanic and also have me thinking. I recently had a hydraulic brake light switch fail, well multiple switches fail and ended up changing over to air.
I also have a breather on my master cylinder cover. It was pointed out to me by a ss member that this is a one way check valve that can cause a vacuum in the master cylinder.
Here is the relativism of these two issues. If the breather created a vacuum in the master cylinder, could that cause a pressure problem with the brake fluid to not engage the switch? By this, we would never know if the switch was bad or good. I hope Cranetruck and other Deuce Genius Club members chime in on my thinking.

If this is breather is the problem, I am going to change over to the vent line just because, even if I have the air switch. When I do change over, does anyone have any pictures of the setup? I have seen it a few times but I don't remember the exactitudes.

Garandfan said:

If the breather created a vacuum in the master cylinder, could that cause a pressure problem with the brake fluid to not engage the switch?

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When I pre-tripped the new-to-me truck, I had the same setup as you (axle breather mounted on the master cylinder cap). The brake lights were not working, although the brakes worked fine with the pedal maybe a tad soft. I decided to bleed them anyway.

After bleeding the brakes, the brake lights worked fine. I tend to believe that I had the vacuum issue. The left rearmost wheel cylinder eventually started leaking and upon disassembly it had both rubber cups "sucked in", which also points towards this issue.

For the time being I left the pressure bleeder connector (male air hose connector) on the MC cap and covered the hole with gauze held by a rubber ring until I figure out a more permanent solution.

I also now have a spare air operated brake light switch...

Just my two cents.

There is a Tech article on how to change the routing of the master cylinder vent, but I can't find it. I would think that if you weren't fording a short length of tubing would suffice. The cylinder should be open to the atmosphere.

I too have a breather cap installed by one of uncle sam's finest.

I do not drive the deuce very often normally I check all the fluids every time I take it out. But it is dark in the garage and hard to see the master cylinder so one day I decided to take the beast out of the garage before checking the brake fluid (more light).

I noticed the first time I did this that I the brake pedal was low and soft. So I get the beast out of it's cave and into the light and check the brake fluid. It looked good so I replaced the master cylinder cap and the brake pedal is high and hard. Since then when I take the truck out of the garage the pedal is low and soft but the fluid level is always good when I check it and once I close up the master cylinder the brake pedal is always high and hard before I head out on the road.

I had been looking at the soft pedal thread for an answer but the brakes are very strong with a high hard pedal except for that initial trip out of the garage. It sounds like this vacuum buildup is the answer.

The brake pedal is low and soft when I start out as there is a vacuum built up from the last trip. I check the fluid level and remove the vacuum. The fluid is level is good (no leaking) and now the brakes are high and hard. I take the truck on the road and slowly build up a vacuum in the master cylinder and then park the truck and drain the air tanks. Several weeks later I start the truck and the pedal is low and soft so once again I check the brake fluid (it is good) and eliminate the vacuum and once again the pedal is high and hard.

In the immediate future I will check the fluid and reinstall the vent tube although it sounds like the best solution in the long run will be a remote reservoir.

My problem is after the truck has been driven for a little while. The brake pedal gets a little harder too. I'd like to hear more thoughts of this potential vacuum effect. I'm gonna put the vent line and see what happens. I'll definitely report back.

Well, now that I read the other posts on this issue, the master cylinder needs to be vented. I think this has has been a cause of my headaches. Every symptom I am having is occurring, hard pedal, brake light switch not engaging after driving for a while, and my grief. Guess I have to work on Big Ugly this weekend. Maybe a remote reservoir with a vented cap?

zak said:

There is a Tech article on how to change the routing of the master cylinder vent, but I can't find it. I would think that if you weren't fording a short length of tubing would suffice. The cylinder should be open to the atmosphere.

Click to expand...

Here you go!

Thanks fellas! Dabtl, I saw your posts on the remote reservoir topic. Excellent ideas! I feel better that I am on the right track with my thinking. I do think the vacuum effect is causing a hard pedal and the brake light not to function. Am I incorrect to think this? Is there a relationship between breather causing the vacuum effect in the master cylinder and causing brake light switch problems and who knows what other problems?