HeadsU.P.

H-m-m-m-m-m-m. NAPA has a new MC for $43? Either they looked up the wrong part or somethings amiss. I bet that is a universal MC. Warning! Stay away from universals. (Should be another 100 for new). Make sure its not only the correct yr but for power brakes too.

I have never swapped out a booster. Others say to pay attention to the rod behind the MC because some yrs were different: shorter-longer-adjustable-threaded.

Vacuum at booster: Put your thumb over the end of supply hose with engine idling. Anything? Few C3s hold vacuum after shutdown because of all the miles of vacuum hose leaks running to & fro.

I think Corvette Central has a MC/Booster combo. It will be the correct one.

san

Your stick shouldnt fall unless somehow it manages to press a little harder and release...
Never had a problem with it.


You should use either brass or nylon like i said,
If it holds pressure, and moves less than a 1/4 distance of travel, then MC is good.

Napa wont know the correct size, any hardware store will have brass or nylon bolts in imperial sizes, buy a couple sizes and try. Better yet, just google the fitting for MC and you will likely find out the correct sizes to pick up.

san

The little fitting that plugs into your booster is probably bad...it contains a check valve and they do go bad at times....this would also explain alot of your problems with your brakes....when vacumn builds up in the booster, the check valve ensures that vacumn remains in the booster during during different throttle position, if bad, your booster wont work correctly because when vacumn flutuates in motor, booster is suppose to remain at a constant vacumn, but if check valve doesnt hold vacumn, brakes will feel spongy sometimes and very unresponsive on hard breaking at other times....

None of that would explain the brake light being on, only thing that causes that is a leak or bad MC. To be honest leaky calipers most of the time wont make light come on either....i would say bad MC and see if you can check your check valve...it should stop airflow in the opposite dirrection of vacumn travel.

HeadsU.P.

Ok, anyone else looking to make some Master Cylinder test lines or homemade MC bleeder kit: The brass fittings mounted on short steel brake lines (premade) can be had at NAPA.

The front port on the master is 1/2"dia with 20 threads per inch.
The rear port is 9/16" - 18 TPI

Thats for a '77 anyway, not sure about other yrs.

Danny76

Thanks for all of that info! I'll be getting to it more this weekend to try and figure out what I'm up against.

Danny76

So I purchased the bleeder kit to use the pieces to find out what size I need and the sizes you mentioned are exactly right. Question....can I purchase flared brass plugs to accomplish this test?

HeadsU.P.

Maybe. I know what you are getting at. A couple fittings is cheaper than two premade lines ($15) from a store. Someone else on here says its do-able. I would use great caution putting any unusual fitting into those ports, because if you use a mirror and look inside the masters ports, its a small flare made for a perfect seal. Sure don't want to damage the MC. That's why I bought one foot premade brakelines, closed off the ends. The bleeder kit you bought will not hold pressure for a MC test run. They're plastic and will blow off with the slightest pedal pressure. Its been said that a brake system has over 400lbs of line pressure. Could be.

Danny76

Yeah, I knew the plastic wouldn't work. My issue is I can't find the darn correct brake line sizes and I really want to figure this out asap so I can order a new MC if needed. That's why I'm almost ready to buy the darn MC and start from there. I'm sure it won't hurt to put new parts on even if I don't need them. It's just money, right?! Lol.

Danny76

I'm kind of thinking outside the box here, but how difficult is it to rebuilt the MC and is it worth it? It would be nice to keep the original on there. I'm just having trouble finding any kind of instructions/videos.

HeadsU.P.

You can rebuild shortblocks, heads, rear axles etc but when it comes to brakes, don't screw around. What if a little kid walks out in front of you? Or you clobber the azz-end of a $100,000 car at a car show?
You can get a rebuilt MC for $75 or new for $130. Like I posted earlier, the workmanship on a rebuilt is only as good as the person that bored the master cyl. If there is any pitting in the bore and you put new seals on the piston, they will likely be damaged. Then, you are back to square one with a MC that won't hold pressure.
As far as you wanting the MC looking original, the new one I got from Corvette Central is a twin. And for a few bucks more, date coded on the unit if you want.
You shouldn't try to cut corners on brakes. There's too much at risk here. You could kill somebody because you saved a few bucks with a cheap kit.

Danny76

No, you're absolutely right. I don't want to skimp on the most important part of the car. My only hold up is I wish I knew for sure this was my issue. I just went to 2 parts stores and 2 hardware stores looking forbrass plugs, brake lines, etc. and I can't find anything in the correct. I just want to order a new MC but wish I knew for sure.

As far as my booster....I did what you said and fired her up, pulled the vacuum and put my thumb over the hose. Idle seems to be about the same. It went way up when I disconnected it of course, but then settled back in with it covered. I pulled the check valve and blew threw one end and it seems to be working just fine. Still, after building up vacuum, turning off engine and then pulling the check valve out, there is no sound at all from the booster. I thought an air gushing sound would show it's working properly? Ugh, and I just hate wanting to do a project and having to wait for shipping. I just want her back running!!!

HeadsU.P.

I can't believe your NAPA does not have short lengths of pre-made brakelines with the correct fittings on one end. 1/2" -20 and 9/16" -18. Most of their stores have racks & racks of brakelines. Then you can block the other ends of the lines with a brass fitting or pinch it off.

My booster doesn't hold vacuum very long either. Not too concerned when its off, just when its running.
Whats your plan on bleeding should you change the MC if you don't mind me asking?
Pedal method?
Motive method?
Vacuum tool method?
Gravity method?

Danny76

Believe it. The first person that helped me didn't even know what it was. I had to pull it up online just to get her to figure out where to look in the stock room. Waste of time.

Well, the first time I bled I started in the rear, farthest away, hooked up a clear line into a bottle, opened it up and kept adding in the MC. I got it really clear and air free, so it seemed to work well, though time-consuming. A good reason to drink beer. Lol.

san

You should only be able to blow through that fitting one way...if you can blow through it in both directions then it is bad. A nylon fitting will hold the pressure testing a MC...it wont hold if you start the car and jam the break as hard as you can, but for testing an MC it will work fine. Furthermore, the tappered flare inside the MC is made of cast iron, i can assure you no amount of tightening a brass bolt will not damage the male side of a flare. Your wrench will strip the head of the brass before you are able to even scratch cast iron.

Someone had mentioned about buying a new MC instead of rebuilding it yourself....i myself couldnt agree more. 90 percent of the time they dont even need honing, BUT, it only takes a little nick in one of the seals to make it fail completely....and it may not even fail until you need it the most!!!

Also...if the check valve is working fine then you need a booster...it should hold vacumn after car is shut off, it is likely leaking and that explains the spongy feel of breaks....the vacumn cant keep up to pedal movement. If the check valve checks out to be bad, get that fixed first. Alot of times when the vacumn leaks in the booster, the breaks feel spongy because the booster isnt boosting the pedal so your foot is doing some of the work making pedal feel stiffer the whole way down. Fix that booster issue first, then go from there.

HeadsU.P.

I went back and re-read your starting question. I had an issue in July much like yours. So here is a little food for thought and to get away from the MC discussions.

Mine pulled to the right (yours left). That means the opposite side is weak. Front caliper was leaking. Swapped it out. Then the copper washer under the rubber hose leaked. (Those washers are only on front calipers) Took it apart, new washer, now no leak there. Still soft pedal after bleeding. Just happened to notice a slightly damp spot where the rubber hose meets the steel line at the frame. No dripping, just damp. New rubber hose, bled again, soft pedal. Days later, one drop of brake fluid on floor. Right rear caliper had internal leak. Never showed up before. So you see, it doesn't take much to loose pressure in the system, not a drop showed up on the floor but still have a soft pedal. And you also have all those fittings at the proportioning valve, a junction block at the rear axle. Look real close, everywhere.

san

Someone did mention in another post about needed to stock a 50 gallon drum of brake fluid if you own a C3...hahaha

Some leaks are so small you dont see drips just like headsup is saying...but only explanation for brake light coming on is faulty MC or faulty porpotioning valve...what it really comes down too like others had stated is safety. Even with a bad booster your brakes should feel solid as a rock pressing on them with car not running....if it feels spongy, like they almost go to floor, you can almost bet its the MC that need replacement anyways.

Danny76

Thanks again guys. I think I'm just going to order a correct MC for sure. After all of this research, I've realized that the one on there isn't original anyways. I'll order one like Headsup said that has the stamps and everything. Trying to decide on the booster. It does look original and it's Delco, so maybe I'll just do it as well. I've read the horror stories about removing that upper left bolt, but maybe I should just get all of the cursing and knuckle bleeding out of the way at once. Most booster's come remanufactured I think?

san

Ive never checked about kits but i remember tearing one apart when i was younger!!!

Danny76

So I just ordered a new MC. Delco, correct stamping, etc. like Headsup said. I figure, looking at the bright side, if this doesn't end up fixing this problem, at least I'll have a new MC that is correct. The one on now isn't the original like I first thought. So next week I'll get this going and go from there. Not going to mess with the booster quite yet. The check valve seems to work, though I don't know if the booster is holding vacuum. So, I'll start and see what happens.
Oh, on another note, the last time I bled the brakes, I noticed the front bleeder screws, when loosened were very loose, almost wobbly. I don't know if there is something stripped inside the threading, but when I would crack it, fluid would drip from the threads which i know allowed air inside. I went to auto stores and bought new bleeder screws, but none fit. I think I now know that 5/16-24 should be the size and I'm going to order speed bleeders that have the insulation/pipe dope on them. I figure I'll do as much as I can at once, thus eliminating "some" of the bleeding I'm about to endure. Oh, and go buy that 5 gallon drum of fluid! Lol.

san

I would order a booster if check valve is ok. Its almost garanteed to be leakimg vacumn...its not that hard to remove booster...alot of guys that work on these vettes that say its hard is becuase they work at this stuff everyday...if your like alot of use you enjoy a challenge....i put my whole dash assembly in already knowing i had heater problems....now dash has to come back out again...i had to see what worked and what didnt before fixing....cant check heater without crusing around the block a few times....and you deffinately need the gauses all installed to test heater....hahahahaha

On a serious note remember this....

Your brake booster is suppose to maintain vacumn regardless of what engine is doing.....imagine driving and booter leak decides to expand a little....right now its only a pin hole....your power brakes will feel lile car is shut off!!!! You can almost garantee your panic brake stoping time will be 50 yeards and 3 seconds instead of 15 yards and half a second. Safety is the most inportant factor when is comes to cars...be carefull doing any test srives until you fix all the problems!!! Drive safe!!!