M101a2 master cylinder question

[wilfreeman]

I picked up my trailer today, pinning the surge brake out of service (I used a 3/8" x 5" grade 8 bolt and locking nut - $.72 at Tractor supply, versus a $3 pin. 4" would have worked). When I got home (about 70 miles) I noticed the master cylinder and the area around it was wet. Upon inspection, it was brake fluid, and it came out of the vent line fitting on the cap. The vent line was broken off at the fitting and missing (wasn't on it when I picked it up). My question is, was this pressure coming out of the vent line because the surge was pinned out? Is the vent line necessary? The TM shows them both ways - with a solid cap and with the vent line.

 

[KsM715]

I bet it wasn't pressure but just the fluid sloshing around enough to spill out.

 

[blybrook]

sounds like something caused the vent line to shear off in the past and you've got some sloshing.

The solid cap has a vent built into the mushroom rather than the hose assembly. You should be able to remove the vent line to cap attachment and put a new hose assembly on there.

 

[quickfarms]

If the vent line was missing for a long time it is possible that their is water in the master cylinder. The water may have caused the leve to rise and the fluid to splash out.

 

[wilfreeman]

Didn't think about sloshing - haven't tested the brakes yet either. Is the vent line like a brake line with a fitting to screw into the cap. I haven't really looked at it in depth yet to see if I can make something or need to buy it. Figured I might put my 4 wheeler in it (for a little weight) when I'm off next week and ride around to check the brakes.

 

[wilfreeman]

Probably did get some water in there, because the vent line fitting is down in a depression, and the line is broken off at the fitting. Guess I need to pull the cap and get all that old fluid out and replace it before I test the brakes, huh?

 

[blybrook]

I would advise to drain the MC, fix the vent and bleed the brakes well with new DOT 5 to ensure you don't have any water in the lines before testing them out. Sounds like you could have a pending rust issue that may come back to bite you later.

I am not intimately familiar with the hose vent style cap, but I believe it would be a brake style fitting threaded into the cap with a hose barb on the other end. Someone with one of these caps may chime in with more detailed information.

 

[wilfreeman]

I checked today and it looks like the line might be pressed in the cap, or maybe just slides in? The broken piece is still in the cap. Is there a replacement cap availabe? I read a post giving replacement part substitutes and it said the master cylinder is the same as a '64 CJ5, so there should be a replacement cap available at the parts store, right? As long as it's vented. Now that I think about it, I think I have one on my M38a1 parts car - it already has a threaded hole in the top if I do! I'll have to go check on that

 

[blybrook]

Yes, a '64 CJ5 master is a direct replacement. I ended up replacing the whole master on my M101A2 and getting a spare cap that way as the local parts house (Napa) couldn't order just the cap. The one that came on the new master is vented with two small holes (~1/32" diameter) at the top; I utilized it for a power bleeder setup. I needed to replace the master due to another issue anyway.

Now that I think about it, I think Croft trailer supplies has the caps available as well; they are also vented through two small holes and not like what you have now.

Good luck!

 

[lindyp38]

my line on my generator trailer is crushed....is it an easy job to replace it???.......must i bleed the system.....?

thanks

 

[blybrook]

my line on my generator trailer is crushed....is it an easy job to replace it???.......must i bleed the system.....?

thanks

It is standard steel line from any parts house. Field bend to fit your application; you may need to get a flaring kit to make the connections yourself if you cannot get the right length line.

Yes, you MUST bleed the system after replacing a line; otherwise you will have trapped air and no brakes. A power bleeder or a mity-vac bleeding gun is your friend on these trailers.

Good luck.

 

[lindyp38]

thanks...........so....even though its an open vent......i still have to bleed the line......that air doesnt escape...?

 

[blybrook]

The vent is for the master cylinder only, NOT the entire system. Air will not escape and cannot overcome the fluid pressure to get back through the master cylinder itself.

Similar vents are on several older vehicles, yet they all are considered closed systems after the master cylinder. I would never change a line on a vehicle and expect that air to escape back through the master. I know that I would not have any *useful* brakes until the system is properly bled.

Yes, it is only a trailer and should not cause much damage if the brakes don't work. I would rather not take that chance and have any and all possible means of stopping should someone or something run out in the road in front of me where I need to make an emergency stop. I am fairly certain that the insurance companies would love to place all blame on the individual with a faulty brake system to get out of their own claims.

 

[wilfreeman]

Definately true - a properly maintained and adjusted braking system on a trailer not only saves wear on your vehicle brakes, it COULD save your or somebody else's life if you have to make a panic stop with a loaded trailer. Any trailer that comes from the factory or aftermarket with brakes that do not function correctly is asking for a lawsuit in the case of an accident - and they will win! Plus it doesn't look good for our hobby.

 

[captw]

M/C cap

just an FYI...in order for the brakes to operate properly, the cap must be vented...I have seen bothe styles of caps used on the M1**A2 series trailers w/surge brakes...mine has the muhroom shaped cap with vent hole, others have a vent line...water tends to get in the vent hole on mine, I'll be converting it to a vent line...W