When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I recently bought a 1967 F100 with a rebuilt manual brake master cylinder and a jammed pressure differential valve.
I rebuilt the pd valve and bled all of the brakes easily to get the brake light to turn off.
My problem now is what I believe to be a leaky master cylinder cap. All of the brakes bleed well, so I'm assuming there isn't a clogged line. The brakes are solid and hold (they're 48 year old drums, so I know they don't respond as quickly as my wife's Scion, but they work well). It looks like the master cylinder was leaking for years, as there's fluid buildup all around there (I think it led to the deterioration of the rag joint), and it definitely seems to be coming from the cap (more specifically, the drivers side of the cap).
I've replaced the mc (the other one was pretty rusty and ragged looking...probably from years of leaks), but the new one seems to be doing the same thing.
My question is, were the original caps vented to allow the gasket/diaphragm to "breathe" a little more? I've read on this forum about vented mc caps, but I don't know if that's what originally would've been on here.
Thanks in advance for any advice.
I had a new MC leaking at the cap it had some crud between the rubber and the steel body. Just had to remove the cap and wipe the cover and mating surface of the MC with a rag.
If it's straight then maybe the rubber seal in the cap is torn or something. Are you actually sure the cap is leaking? If so you should be able to see from where.
OK so I must have glossed over your original question.
I'm not expert. This is what I think from experience. The rubber seal under the cap expands downward as far as to invert itself. It does this as the brake shoes wear down and the fluid level drops. No vent.
Seems like I remember maybe Orich and maybe Baja talking about this before. Seems like Orich said he got a aftermarket replacement chrome cap and that solved the problem.
OK so I must have glossed over your original question.
I'm not expert. This is what I think from experience. The rubber seal under the cap expands downward as far as to invert itself. It does this as the brake shoes wear down and the fluid level drops. No vent.
Mine is not vented. If I parked in my driveway with my front toward the road, it was slightly uphill. The cover would leak at the back of the MC. If I parked facing downhill, no leak. I flat filed the top of the MC which helped some. I then put a shim on the cover retainer where it touched the cover and my leak is gone.
I went back out to my shop to take a closer look at some of the old conventional style Ford cast iron MCs I have in a box.
The lids on the MCs that didn't have obvious vents in the domes still have vents in them.
This is the lid that came off the cast iron MC I had put on my truck, prior to installing a modern cylindrical aluminum body/plastic reservoir MC.
Notice the raised stamping in the lid (the areas where the three picks are pointing to). These are air channels to allow atmospheric air to the topside of the seal.
The air enters into the channels from these two places on this lid.
This is the crusty MC that was on my truck when I bought it. No obvious vents on the topside of the cap but, what do you notice where the picks are pointing to?
I don't post in this forum but this was the lead thread and caught my eye as I recently experienced the very same issue.
What eventually worked for me was removing the master cylinder and working the top edge across a large piece of emery cloth until the edge was nice and smooth.
I also took a pair of large pliers and bent the retaining wire to apply more downward pressure on the cap (it now requires a screwdriver to remove).
It's been nearly a month now and the master cylinder has remained leak-free!
I don't post in this forum but this was the lead thread and caught my eye as I recently experienced the very same issue.
What eventually worked for me was removing the master cylinder and working the top edge across a large piece of emery cloth until the edge was nice and smooth.
I also took a pair of large pliers and bent the retaining wire to apply more downward pressure on the cap (it now requires a screwdriver to remove).
It's been nearly a month now and the master cylinder has remained leak-free!
Ah yes I had gotten a new MC from rock auto which, I regretted due to turn around time and will never buy online brake cylinder stuff again.
Made in China with the funky side lip vents that leaked. I flattened them
re-bent cap clamping wire that was weak and didn't not hold the cap tight and still leaked. Like Jeff said I found a chrome one with the vent in the upper part of the lid. So got the good new replacement at guys car show and using the china rubber cap gasket with no leaky.
Man I feel your pain in the AZZ trying to stop it from leaking, not one drip since changing the led.
Orich
