85 300D Brake Failure
 

Pretty much a total failure. Pedal has been low for a while but today it went just about to the floor before any brake action took place, a lot of effort required, to the "scared-as-hell" stage. Fluid reservoir was empty but no brake light. I refilled the reservoir and tried pumping the brakes looking for at least a little pumping up action but literally no change. I had my wife looking under the car for obvious leaks but nothing was seen. Although I haven't messed with the brakes much I did put new rear pads in last year and checked the fronts. If brake fluid were being sucked into the power brake can, should I see a lot of white smoke out the tail pipe or it this totally different on this car due to vacuum pump?
Since it is not pumping up, does this sound more like a Master Cylinder failure?
Should I just spring for a new MC and try that before replacing all calipers? Is there someplace here that explains a bit the process of "bench" bleeding? The brake light not coming on, can I get a new plastic reservoir for the MC too?
Thanks for any advice.

Sounds like MC failure. You'll probably find that you need a new vac booster as well.

Should I just spring for a new MC and try that before replacing all calipers?

Yes


Is there someplace here that explains a bit the process of "bench" bleeding?

MC should come with fittings and little tubes, attach the reservoir, hook up the tubing and return it to the reservoir, fill the reservoir and start pumping. I typically hold the MC and push on the rod. Do this while watching the fluid returning to the reservoir and when it has a good flow you should be good to go. Use caution and don't spill any on the paint work. It will remove paint.

Replace the reservoir and the rubber bits used for the seal and the little pushbutton thing on top if it has one.

I just had the exact same failure on my 77 300D. New Master Cylinder fixed it. I used a new ATE, not rebuilt.

I bought a rebuilt MC for both my 300D's at Autozone (~$40), but they don't come w/ reservoir. You can buy the rubber grommets at the vendor here. The reservoir level sensors (2) are a simple 2-wire on-off switch, so can check w/ a multimeter. You should hear them rattle as you shake it. If no rattle, try cleaning by pouring in ethanol and shaking around, then completely dry it. If the brake light wasn't coming on, the bulb may be bad. The rubber caps are resilient for testing. Push down and level warning should light.

Your booster if probably OK. I had brake fluid in both mine, so removed and poured out & absorbed w/ rags. I now use silicone fluid, so no more corrosion. No white smoke, the fluid would just go out the vacuum pump, but I can't see the the level reaching the vac port.

More than likely, the fluid was pumped into the brake booster.

Check then empty if necessary, new master cylinder time.

This is actually a partial failure, due to a split braking system you still had either front or rear brakes.

Unbolt the MC from the booster and look for liquid. There is never enough brake fluid to make smoke on a gas car that uses engine vacuum for the booster. On a vacuum pump car, that won't happen either even if the pump outlet is connected to the engine air intake.

And, 30 lashes for driving with low brakes. . .

{Chuckle}

Not necessarily. He could have been driving with the partial failure (low pedal and no brake warning light), and this was the failure of the part of the system that was still working.

Yeah, up until this failure, low but functional pedal. The MC, there some kind of seal/gasket between the MC and the vacuum thing. Would that seal be part of the MC replacement or do I need to order that separate? I'm semi-screwed now since it is getting cold but my basic plan it to have all the parts here in case of a decent mid-winter day or rent a heater for the garage.

The MC has two pistons, one seal can fail and leave the other half working. I replaced mine when it failed this way, not too long after completing some brake repairs and thinking, 'oh, I was only supposed to pump the pedal part of the way to bleed the lines, wasn't I?'. I probably tore the seal on a wear ridge on the cylinder bore.

So you all don't replace the booster when this happens? Good to know.

As far as the calipers, peach parts here sells rebuild kits for cheap. Oreillys sells a hone tool to clean out the inside, and I use fine sand paper on the piston.

I always rebuild my calipers when unknown. The auto parts stores aren't carrying the kits as often, they want you to buy the whole unit. But it's a really easy process.

I'm going to be doing this in the next few days and will post it.


Sent from my iPhone using Tapatalk

Bench bleed: I don't do them on the bench, this method will work fine with the new MC installed in the car.

I bought some cheapo fish tank air hose at wallyworld, and ran a piece from the front left wheel slave cylinder bleed valve up to the MC reservoir opening. Pumping the pedal resulted in fluid flowing from the MC through the brake line and wheel cylinder into the hose, then back into the MC reservoir. When there were no more bubbles visible in the clear hose, close the bleed valve. Next run a longer piece of hose to one of the rear wheel slave cylinders, and repeat.

When you're done, you have both bench-bled the MC and also bled any air from your brake system that might have entered when you removed the old MC. Note that if your MC has separate connections for each front wheel, then you will also need to bleed air from the right front wheel brake circuit.

From the symptoms the low pedal was due to low fluid / air in one circuit. When this circuit finally emptied, the working circuit is all that stopped the car.

On a dual circuit master cylinder, there is a free floating piston ( 1 ) towards the closed side of the bore and another ( 2 ) attached to the brake pedal. When everything is working, the pedal pushes piston 2 down, a column of fluid pushes piston 1.

If there is a leak on piston 1, it must bottom out before piston 2 can make any pressure. If a leak on piston 2, it must travel far enough to physically contact piston 1. This is why the pedal will have a to the floor travel. The very hard pedal when you get there comes from 1/2 the brakes trying to stop all of the car.

There are bound to be online vids of how this system works.

Loss of rear brakes isn't less of an issue than fronts as most braking occurs in the front. ( weight transfer )

Don't know why my interior brake light didn't work. Pushing each of the two manual "testers" worked. When I do the MC I'll have to play with the reservoir and try to clean it out. Does the booster come out semi-easy? 4 Nuts inside? Or, if fluid did actually leak into it, does the vacuum pump suck it out? I'm semi sure I still have power brake action as it hisses when I press the brake pedal. I'm thinking that removing the booster might be a lot of work and am further thinking just swap the MC an proceed from there with fingers crossed. I am 90% sure I wrecked the MC from pushing the pedal too far after installing ne rear calipers a while back. Pick-n-pull around here has not a whole lot to choose from, reservoir seems to be difficult to source.