possible master cylinder problem

jmohn · jmohn's Garage

Winter's finally over so I got the car ('78 SC) uncovered yesterday, took a short drive and found the braking very poor, way too much pedal travel and won't really "pump up". Later I started the car from cold with my foot on the brake pedal, the pedal goes down slightly when the engine starts (and then keeps going down - not good), so I don't think the vac. booster is the problem. I find no leaks in the system, so my diagnosis is the master cylinder is leaking internally (piston seals).

Does anyone have any other possibilities/suggestions before I order a new master cylinder.

Thanks,

Jerry M
'78 SC

wrinkledpants · 04-29-2013, 07:06 AM

I would probably start with a bleed before replacing the MC if you don't find any leaks. I mean, it seems weird that you covered it last season, and it worked fine, but now it doesn't. At least if you bleed it, drive it, and if you do have a leak, it'll likely reveal itself. In a system that has positive pressure compared to ambient, I think it would be rare that you'd be taking in air without releasing fluid. It's certainly possible, but most of the time I think you'd see a leak.

Targalid · 04-29-2013, 07:08 AM

Jerry,
My only suggestion would be to bleed the brake system before you spend money on parts. Bleed it, replacing the old brake fluid as you do so, and look for problems. A complete inspection during the bleed process may uncover the problem. It's cheap and you should replace the fluid every year or so anyway.

kerthunk · 04-29-2013, 07:14 AM

My master cylinder was leaking, but it wasn't obvious as it was dumping the fluid into the booster.

dave 911 · dave 911's Garage

Do you have any brake fluid under your pedal cluster? That was the telltale for me that my master cylinder was leaking.

Jdub · 04-29-2013, 08:07 AM

What John said - missing fluid with no discernable leak area can often mean leaking into the brake booster.

But don't go there yet. Siphon out all the fluid in the MC reservoir, put in fresh, and let the manual bleeding begin!

DRACO A5OG · DRACO A5OG's Garage

Check the t fitting behind the stone guard, mine was weaping from there.