Master cylinder?

[zxcvbob]

The brakes went out on the '94 Cadillac.  They were working fine, and then they weren't.  The computer said "Check Brake Fluid" and I did and the reservoir was empty.  I haven't seen any signs of a leak anywhere, so I filled it up with brake fluid but didn't bleed the lines -- I hoped a bubble hadn't got past the master cylinder yet.  So I pumped the brakes slowly with the engine off and they go way too easily and I figured "crap, there's a leak".  But I looked under the car and no dripping fluids, and I looked in the reservoir and it was still full.  OK, the master cylinder is just still full of air, so I pumped it very slowly a few more times and it seemed to be getting better, barely.

I started the engine and put it in reverse and slowly backed down the driveway, hitting the brakes every couple of feet.  Not good, but not that bad.  Then I went forward the same way.  When I got to the end of the driveway I stepped on the brake hard and it kind of popped and the pedal went down another inch (I thought it was bottomed already) and no brakes at all.  I was just barely rolling, and I stopped by putting it in neutral and using the foot brake.

I checked the fluid level and it had dropped a little but not much.  WTF?  It kind of felt as if one brake line had burst yesterday, and it was still operating on one circuit after I refilled the fluid and then that line burst when I was playing with it.  But if any of that is true, where is the fluid going?

That's why I think it's the master cylinder.  Not that I'm gonna do much with it until spring.  The car is worth less than its scrap value, so I don't really want to put any money into fixing it, but last time I replaced a master cylinder it was pretty easy and didn't cost a lot.  That was 25 or 30 years ago...

Maybe I should loosen the 2 brake line fittings going in the master cylinder (one at a time) and bleed the MC?

[Ben]

Sorry, couldn't resist.

[Brad Johnson]

You need to fully bleed the brakes before making any presumtions. {Pumping the pedal a couple of times isn't going to do it.

If it's the master cylinder leaking there will be a huge blotch of brake fluid residue (not to mention peeling paint) under it in the engine compartment.

Brad

[zxcvbob]

If it's the master cylinder leaking there will be a huge blotch of brake fluid residue (not to mention peeling paint) under it in the engine compartment.

Unless it's leaking into the vacuum assist...

(I hate bleeding brakes in the snow)  

Ben, that looks just like it!  You must have a copy of the service manual.

[Brad Johnson]

Unless it's leaking into the vacuum assist...

Which will usually result in a huge cloud of smoke every time you crank the engine as it pulls fluid up into the intake via the vacuum line.  That, or it will be drooling fluid into the passenger compartment, making an oil-soaked mess of your carpeting.

Like it or not, you're going to have to do a full system bleed.  Even that may not get the job done if there's air in the ABS module.

Brad

[White Horseradish]

I had this kind of thing multiple times on different cars. It probably is not the master cylinder.  What is likely happening is that you have a leak in one or both rear wheel cylinders. The brake fluid soaks into the brake shoes and burns up on braking. Little of it gets outside the brake drums so this does not result in noticeable puddles.

So, I'd yank the rear wheels and look inside the drums. You will likely find a soggy mess.

[zxcvbob]

That sounds plausible.  (also sounds like it can wait until April, I just need to start the car and idle it a couple of times per month until then) 

Do either of you remember offhand what size wrench a GM brake bleeder takes?

[Tallpine]

One cylinder to break them, one cylinder to rule them, and in the darkness bind them.  

You need to fully bleed the brakes before making any presumtions. {Pumping the pedal a couple of times isn't going to do it.

If it's the master cylinder leaking there will be a huge blotch of brake fluid residue (not to mention peeling paint) under it in the engine compartment.



Brad

Totally correct on the first point.


If the master cyl piston seals are bad, the fluid could just be leaking past back into the reservoir.  IOW, the m-cyl really isn't pushing very hard out to the wheel cyls.

You could have had minor fluid loss over a long time if you hadn't checked it.

Unfortunately, the master cyl seals could have dried out from sitting around without fluid  


One time I had some wheel cyls replaced, and the master cyl failed almost immediately (almost had a wreck coming home from the shop).  It was ok before but just bleeding the system put it over the edge  

Do either of you remember offhand what size wrench a GM brake bleeder takes?

Vise-Grip 

[White Horseradish]

Do either of you remember offhand what size wrench a GM brake bleeder takes?

Sometimes it's 10mm, sometimes, something else. I just check every time.

[mtnbkr]

If the master cyl piston seals are bad, the fluid could just be leaking past back into the reservoir.  IOW, the m-cyl really isn't pushing very hard out to the wheel cyls.

This.  I had an accident in 2003 because of this exact thing.  There was enough brake to stop in normal day to day traffic, but when I had to execute a panic stop in heavy traffic, the brakes didn't have enough squeeze to stop me. 

The car had undergone VA's state safety inspection less than a month prior. :(

Chris

[seeker_two]

Sometimes it's 10mm, sometimes, something else. I just check every time.

I'd always thought .357Mag worked on everything......

[BReilley]

That sounds plausible.  (also sounds like it can wait until April, I just need to start the car and idle it a couple of times per month until then) 

Do either of you remember offhand what size wrench a GM brake bleeder takes?

8mm, I believe.

[KD5NRH]

Do either of you remember offhand what size wrench a GM brake bleeder takes?

Six inch usually works fine 
http://amzn.to/ugnOfr

While the little cup type bleeder kits aren't bad, I'm a big fan of the $30 hand-pumped vacuum type, just for the advantage of not having to go mash the pedal every time, plus the cup never seems to be big enough, so it's pump the pedal, top off the cup, start over because it sucked air back in when the cup splashed empty, etc.

[Regolith]

Six inch usually works fine 
http://amzn.to/ugnOfr

Never been a fan of those. I can never get them to stay tight enough, especially for really difficult nuts.

[Brad Johnson]

While the little cup type bleeder kits aren't bad, I'm a big fan of the $30 hand-pumped vacuum type, just for the advantage of not having to go mash the pedal every time

I finally got smart and picked up one of the kits you're talking about.   Sure does make things a lot easier and cleaner.  The only slight PITA is having to crawl out out from under the vehicle and top off the fluid reservoir to keep it from going dry (if you're doing a full flush).  The new pressure bleeders are even better than that.  You pressurize the system with a pressure cap attached to a bottle of fluid.  Attach it, pressure it up, and crack the bleeders until the fluid runs clear.  The fluid in the pressure bottle automatically replenishes the MC reservoir as the system is flushed.

Brad