brake master cylinder

[minifridge1138]

My brake master cylinder needs to be replaced. I'm in the middle of moving and my aim and service manual are in a box someplace.

Do I need to anything to prep it before installation?
I've seen posts talk about bench bleeding before.
Do I need to do that?

Thanks!!!

 

[petes74ttop]

Yes you have to bench bleed the new M/C, the bleeder hoses come with some of the replacement M/C's PG.

 

[JohnZ]

You'll find a bench-bleeding kit for about $5.00 at your auto parts store in a blister-pack in the "Help" section. Clamp the master in a vise, install the fittings and hoses, fill the reservoirs with brake fluid, and stroke the rear piston with a large phillips screwdriver until there are no bubbles coming out of the hoses. Then replace the rubber hoses with rubber caps, install it on the car, and connect the lines, then bleed the system.

 

[Crimson Thunder]

I like to bench bleed the MC on the car.It becomes a two person job that way(someone to press the brake pedal while you watch the bubbles)but it seems to make less of a mess.

 

[JohnZ]

I like to bench bleed the MC on the car.It becomes a two person job that way(someone to press the brake pedal while you watch the bubbles)but it seems to make less of a mess.

Doing it from the pedal is fine if it works for you, but the pedal usually won't stroke both pistons to the end of the bore, which can still leave some trapped air. Doing it on the bench, you can always get full stroke of the pistons and drive all the air out of the bore.

 

[Crimson Thunder]

Hmm..That makes sense.When i was a teenager, my friend and i were changing a MC on his 69 Camaro when his father came out screaming at us that we were bench bleeding it wrong and showed us the "on the car" method.I've been doing it that way ever since.

 

[TimAT]

Doing it from the pedal is fine if it works for you, but the pedal usually won't stroke both pistons to the end of the bore, which can still leave some trapped air. Doing it on the bench, you can always get full stroke of the pistons and drive all the air out of the bore.

100% correct- but I found out when I replaced the M/C on my pickup- It has a new style cast aluminum M/C and they don't hone the bore the full length like they used to. I bench bled it like I've done forever, and cut all the seals in it. End results were not good. And another M/C to boot. Not an issue for the C3's, but something that everyone needs to be aware of.
Sorry for the hijack..

 

[minifridge1138]

No worries about the hijack.

That actually happened to me.
I just put ANOTHER master cylinder in.
Three in 2 weeks.

I've still got a soft pedal....

 

[TimAT]

You still have some air trapped somewhere or a flex line is giving up. I did the brakes on my 69 last winter- rebuilt the calipers with O-rings, new stainless steel flex lines, and the M/C rolled over and died. New Wagner M/C and bled about a gallon of brake fluid thru it. Still was not happy with the feel. I got a Motive Bleeder, ran less than 1/2 gallon of fluid thru the system and the pedal is great. Not 100% sure if it was the bleeder or just the additional fluid thru the system, but I'm going to bet it was mostly due to the bleeder. I've since talked to several other guys and all agreed the Motive Bleeder makes the difference.

 

[Evolution1980]

I just put ANOTHER master cylinder in.
Three in 2 weeks. I've still got a soft pedal....

Ummm, then maybe it's not the master cylinder??? :confused

Did you check your proportioning valve? All the brake lines? All the calipers? If/when you bleed the brakes, there are plenty of threads around here about the proper way to do it. In fact, I think we have a write up in our technical archives.

EDIT: I didn't find exactly what I was looking for, but this link should get you started.

 

[minifridge1138]

The original master cylinder started draining fluid from the rear reservoir where the MC bolts to the vacuum booster. Brakes were firm as long as I kept the reservoir full.

The first replacement installed fine and the rear wheels bled normally. But the front wheels were sucking air from the MC, not fluid. We'd pump the pedal until we got pressure, open the valve, out came fluid, close the valve, let the pedal up, and then the pedal had no resistance. Could not build up pressure. Only air came out of the bleeder valve.
Let the car sit for 2 minutes and the pedal got firm. We'd get 1 bleed of pure fluid and then just air.

The 2nd replacement installed fine. Bleeding was normal, but the pedal didn't get as firm as the original one.

I'm going to try bleeding again later (perhaps after I buy the power bleeder).

Thanks for the link.
I'll double check and make sure I wasn't making any obvious mistakes.

 

[Evolution1980]

Thanks for the link.
I'll double check and make sure I wasn't making any obvious mistakes.

The link has expired. All I did was Search -> Advanced -> keyword: bleed brake and kept the search limited to just the C3 forums.