Master Cylinder Bore

-
norton

norton

Doing the best that I can
Joined
Jul 30, 2013
Messages
875
Reaction score
163
Location
Etobicoke, Ontario
I need to get a Master Cylinder, for my 1972 Duster. This is a factory H-Code car, with manual drum brakes all around. Every supplier site that I look on asks about the Bore. Is there any way of knowing this without removing the old Cylinder. The car stops good with the old one, I just can't seem to get the lid to stop leaking. I replaced the gasket with a new one but it leaks even worse.
 
norton said:
I need to get a Master Cylinder, for my 1972 Duster. This is a factory H-Code car, with manual drum brakes all around. Every supplier site that I look on asks about the Bore. Is there any way of knowing this without removing the old Cylinder. The car stops good with the old one, I just can't seem to get the lid to stop leaking. I replaced the gasket with a new one but it leaks even worse.
Click to expand...

On several occasions like yours, I found the brake system to have air in it, and when the brake pedal is released, there is a violent fountain of fluid shoots up against the cap, and some migrates under the cap. I know some that should know better will disagree, but that has been my experience, and bleeding the air out of the system cured the issue, not a new seal, top, or M/C.

Want to check it, have a friend pump the brakes a few times and hold the pedal down, THEN remove the cap and have the pedal released, if you get a prolonged fountain of fluid squirting back into the M/C, then you need to stop the fluid return, by adjusting brakes, and bleeding.. jmo

PS, have plenty of water nearby, to immediately rinse any spilled fluid off the paint, as brake fluid is excellent paint remover.

hope it helps
 
According to my research 1 inch is what the car came with. And my research has been extensive.
 
Smaller bore is generally preferred, since an easier pedal. I agree w/ post 2. The geyser of fluid is an indication of air in the system, or soft rubber hoses that are ready to burst, so fix that first (if you see it). On my manual drum Valiant, I use an MC for a 95-99 Breeze/Stratus/Cirrus w/ ABS on a 2-4 bolt adapter plate ($30 ebay). MC is aluminum, 7/8"D bore, w/ the modern plastic reservoir that seals tight & has a low-level warning switch. But, many later MC's can be used w/ the adapter plate. Many here use one for a late 80's Dodge truck w/ manual brakes.
 
Thanks guys, I'll check the flex hoses and bleed the brakes, before looking into a new master cylinder.
 
Just wondering if you cleaned the sealing surface on the master. They can get pretty rusty and rough. It fixed my leaking problem.
 
I know this is an old thread but did bleeding/adjusting help in thius situation as I also have a leaking issue from the lid that I just cant seem to sort?
 
We don't fuss "old thread" here. It is better to keep info in the proper post, regardless of date.

Leaking lids are a common problem w/ old cast-iron MC's. If you can correct a "fluid geyser" problem, that helps, but a good lid shouldn't leak even with splash. If it leaks fluid out, it lets moisture in, and that causes internal corrosion with normal glycol fluid. I understand there is some humidity in the UK. That is one reason to change to a modern aluminum MC w/ plastic reservoir. But you will lose that traditional look of a rusty MC and rusty firewall from dripping fluid (great paint remover). If you prefer 1-stop shopping, Dr. Diff sells a package of 2-4 bolt adapter plate and MC, the later for a 1980's Dodge truck. If manual brakes, insure the bore is small enough for your leg, especially if you have front disks. Professional football players ("Rugby" for American colonists) can get by with a larger bore MC.
 
Last edited:
I amend my post #9. In another thread, Dr. Diff responded that their 15/16" bore MC is special made for them, meaning you can't just buy one for an 80's truck at an auto parts. I don't know. Rockauto lists the bore dimensions of many of the MC's they sell.
 
this old post describes my problem to the letter. definitely getting fluid between m/c cap & rubber seal. there aren't too many problems these near 50 year old mopars have had that are not listed on this site. my rear pot is the only 1 that drains so maybe its the frt brakes that will need the attention. helpful & informative. thanks. mike.
 
Brake fluid is very creepy. It loves to creep, and will move thru the tiniest of openings.

I agree with the geyser-talk. With drum brakes, a geyser is somewhat normal, because the wheel cylinders always park in the same spot. And if the self adjusters are not working 100% then eventually you are pumping more and more fluid down the line, and it all comes back when the return springs do their job.
Of course this takes more and more pedal travel to get the job done and so, one day you say what the heck?, and then the lightbulb in your head goes on,lol. Sometimes the rear shoes haven't been rubbing the drums for .......a very long time.
 
Last edited:
I don't think I have seen an old Mopar that didn't have a rusty firewall below the MC. Glycol is a great paint remover and one reason many classic car owners use DOT 5 silicone fluid (that it doesn't suffer internal rust).
 
You must log in or register to reply here.
-

Back
Top