Changed master cylinder-brakes won't bleed.

I've been reading these forums for the last month or two and you've inspired me to get my 1971 250 back on the road. This is a very helpful board. Inspiring, in fact.

Well I've worked out the cooling system problems, and now I've gone on to the front end and brakes.
The car was pulling to the right when I broke, and I discovered the tie rods and drag link were shot. I replaced them along with the steering shock.

The brakes were siezing, so I've changed the master cylinder along with the brake hoses. This is where I'm running into trouble.

I have no pedal pressure, and it sounds like air is getting into the system. The left rear doesn't get any fluid through the bleeder valve when I open it, while the other ones bleed just fine.

I'm probably missing something quite obvious, and would appreciate some input on this. I feel I'm really close, and I can't wait to get her on the road again.

Thanks,

Matt

[psfred]

Matt:

Ah, brakes on neglected MBs. My favorite subject (not!). I've fixed so many of these it's not funny anymore.

First, seizing calipers isn't the master cylinder, sorry to say, it is letting the rotors wear down too far, usually, so that the pistons get cocked in the bore of the caliper when the backing plates hit the anti-rattle spring. This makes them drag, and roasts the dust boot, and then crap gets in between the piston and the bore above the seal, and they stick.

I'd recommend a set of caliper rebuild kits, do all four wheels, it's not hard. Replace rotors if they show a distinct lip at the outer edge or it the backing plate on the pads hit the antirattle spring on the last set of pads you had in there. They are only $60 or so each, but a pain to replace. You have to repack the wheel bearings (or replace them, to, typically, since the idiot PO who let the rotors get too thin also had a cheap brake job done, and they grossly overtightened the wheel bearings).

The rebuild is pretty fast -- remove caliper (use an 11mm line wrench on the line, leave the line on the steering knuckle or rear axle housing, it's faster, believe me!), the use compressed air to pop one piston out at a time (use a C-clamp to hold the other one in). Pry off dust boot and head shield first, then lift the seal ring out with a sharp pointed tool -- I recommend something plastic or brass unless you have a hooked scribe set that will allow you to stab the point in without any danger of getting the steel point into the bore surface.

Clean it all up -- don't use any abrasives, please, except some scotchbrite on the OUTER bore -- above the seal ring -- brake parts cleaner works fine. Get all the crud off the caliper, too, you don't want any in the bore!

Coat the new seal with brake fluid and insert in slot. Set the new heat shield on the piston and make SURE it is correctly aligned -- you cannot rotate once it is in place without the special tool -- and carefully push the piston back in. A short piece of flat wood is usefull here. Make sure you have plenty of brake fluid on the piston, you don't want to drag the new seal over and cut it. Pop the new dust shield over the piston and tap down on the outer edges. The piece of wood is handy here, too. Make sure it is seated all the way round, or dirt will get in.

Press heat shield down into place, make sure the ridge on the piston fits through the slot or the brakes will squeal. You cannot remove it intact once you put it in, be carefull.

Repeat for other side, then for other calipers.

If you cannot bleed the rear brake, completely remove the bleeder (it just screws in). Most likely it is plugged, you may need to gouge out the crud. YOu can also use your handy MitiVac to bleed the brakes (this is VERY nice, by the way!).

Brakes should bleed down by gravity if the lines aren't empty. That one may simply have so much air in it you haven't gotten it out yet, too -- took forever to get all the air out of the brakes on my brother's Chevy 1-ton!

Also, you might want to consider replacing all the brake hoses. They are cheap, and if yours have been in there for 30+ years, they will surely leak or flake off on the inside.

Peter

Thanks Peter- I'm hoping that it is just a matter of getting all the air out of that circuit. The caliper rebuild sounds pretty in depth.

The rotors were changed by me 4 years ago, and the car hasn't been driven for three, so they're in good shape-along with the pads. I'm hoping the brakes won't stick once I get that last wheel bled and road tested.

Also, I didn't change the rear hoses, so I'll change those before I proceed.

One more point- It seems air gets back into the system when I pump the brake pedal. There's a new O ring installed with the master cylinder, but it sounds like that is the area where there is an air leak. Is there a seal in the booster I've overlooked?

Thanks Again-I'll keep you informed of the progress.

Matt

[Mark DiSilvestro]

That 'O' ring keeps the booster airtight. It should have no effect on air getting in the master cylinder. The hydraulics are sealed off from the vacuum system by seals in the master cylinder piston. Did you 'bench-bleed' the master cylinder before you connected the lines to it? Definitely replace those rear brake hoses.

Happy Motoring, Mark

[Clunker]

If the car has been sitting that long it could very well be the calipers sticking. I had to drive my old 115/240D for a week before the brakes "got loose." Do the brakes feel like they are sticking in the rear? I had rust on the rotors/pads. I think it was the only occasion when the old: "aw just drive her thats all she needs" adage actually worked.

Well, well-bench bleeding the master cylinder. I didn't remember to do that. Thanks Mark, it sounds like that may be my problem. I'm heading up the coast to Sonoma in my other car (a BMW 325ic) tomorrow, so I won't get to this until next week. I'm assuming now that everything is put together it is just a matter of pumping the pedal until I get all the air out. At least I'm hoping I don't have to pull the master cylinder out again. Thanks for all the input. I'll keep you posted of the progress.

And yes Clunker, the rotors have rust on them and I'm hoping that everything will run smoothly once I get it on the road for a week or two. I just haven't gotten that far yet.

Thanks Again,

Matt

[psfred]

Matt:

You might want to pull the pads and make sure they are not rusty -- it can make them stick in the caliper and drag even though the calipers are fine. My mechanic recommends standard antiseize on the back of the pad and on the sides of the backing plate.

Chances are that you have air in the master cylinder -- quickest way to get it out is to gravity bleed the brakes -- just open the bleeders and let them run while keeping the brake fluid topped up in the master cylinder. Then bleed each one, you will get all the air. Rear brakes won't gravity bleed unless you have fluid in the lines.

Peter