'73 450SL Master cylinder

[Salado]

Hi -- I'm a newbie here so be gentle.

Whenever I'm driving around I notice that the brake pedal gets mushy. Pump it a few times and it comes back but then degenerates again. I stopped on a small incline the other day and the pedal slowly went to the floor.

I've checked the lines(no leaks), pads & rotors(within acceptable limits based on info from another forum) and have bled all the wheels. Still the same thing. So I'm beginning to think it's the master cylinder bypassing internally.

Would you guys recommend a rebuild or just biting the bullet and putting in a new MC? I've had good luck rebuilding other MCs, but not sure about MBs.

Any advice would be appreciated.

[speedy72t]

I've never had great luck with rebuilds myself. so I usually go for new. Depending on the year it seems they run from $100 for the late model version to about $220 for the earlier versions. Not cheap but I'm looking at a new one as well.

[230/8]

I would certainly recommend checking all four corners and everything in between before replacing the master cylinder. No problem with replacing it, just make sure you have identified the real culprit. Afer all, your goal is to get the brakes fixed, right?

You might actually discover that nothing is leaking. The pads could be worn to the point that the master cylinder has sucked air and all you need is to replace pads, change the fluid, and bleed the system.

230/8

[speedy72t]

well based on the fact that he bleed the system and checked all four corners it certainly does seem that the master cylinder is a good bet. Especially considering the very classic systems of a blown master cylinder.

Do check the rubber lines going to the caliper though. It's pretty likely that they are shot as well. If they are over ten years it's time to replace them and if the system is going to be apart now is really the time.

P.S. In humble opinion only

[Salado]

Thanks all for your input. I have to agree that a new MC is probably the way to go. About the cheapest one I've found online is about $209.

speedy72t-- I did check the lines but will definitely do it again. What a tight squeeze!!

Basically, I think about everything on this SL is original, so it probably would be a good idea to replace the lines as well. I know the tires are original (83,000 miles on the car) and they're next on the to do list. I was driving about 70 mph last week and started thinking--"What are you doing?" you're driving 70 mph on 30 year old tires!!! They may look good still, but they're still 30 years old!!!

Anyway-- thanks again-- a great site.

[speedy72t]

what happens to the rubber lines to the calipers is more of an internal failure. they swell up and the fluid can't move freely back out of the caliper. Among ther issues. My point is - replace them. The transmission cooler lines are another big issue on these old benzes. there are pretty decent pressure and when they go it's a serious mess. The master cylinder, four brake lines and two cooler hoses should run about $300

[Salado]

OK-- more items for the to-do list--cooler hoses.

One more question please. Do you know if the MCs for this car come with a bleeder kit and adaptors? I think I've read elsewhere on this site that a bench bleed is imperative. Don't mind doing it just need the adaptors and hose to do it with. Hate to have to buy a bleed kit-- they seem expensive.

[Wittich]

It is absolutely important to "prime" the mastercylinder befor installation. I usually clamp it in the bench-vise, take a turkey-baster, yes you read right, fill the MC with brakefluid and activate it once with a large screwdriver. Hold it on the in-stroke, cover the outlets with your fingers so it will not suck air in when you let the scewdriver return. repeat that procedure untill brakefluid squirts out freely on every "push-stroke" Plug the outlets of the "filled" MC with the plastic plugs it comes with, while holding the screwdriver with your belly, you need both hands to plug the outlets. the idea is, that you have the MC full of fluid and not air when you are installing it.
After bolting the MC in place, fill the reservoir with fresh fluid, connect the lines, making sure the reservoir is toped-up ALL THE TIME. Yes, it is very messy so have plenty of rags ready to absorb the spilled fluid.
I have found that this procedure does not allow any air to get into the system and nine out of ten times you don't even have to bleed the system after.
Hope that this is of some help.....

CHEERS !
WITT !

[speedy72t]

There is a home pressure bleeder out there for about $50 that's basically a weed killer sprayer adapted for our purposes. www.germanautoparts.com has it on the right hand side of their web site. I'm sure others do as well. I made one for myself before I realized I could by it for about $6.00 more than it cost to make it. the only caution is to keep the pressure under ten pounds, if you go that route. I sent the top half of the resoivor on my Audi winging past my head at a high rate of speed at 15lbs.

pre-priming works, but can be very messy. This method isn't totally clean but it's a nice way to be able to completely flush your brake system every year or two with a minimal about of fuss. The problem wit the two man pump the pedal routine is that you will often destroy an older master cylinder this way by pushing the piston way past its normal travel position. Been there, done that, use this one man pressure bleeder for everything brake related now.

[Salado]

Thanks for the hands-on tips there Witt. I will certainly keep that in mind. I've read where some MCs come with threaded adaptors and hoses. You screw the adaptors into the ports, connect the hoses and run them back into the MC so when you manually stroke it, all the fluid goes into the MC and not on the floor or you.

speedy72t-- checked out that pressure bleeder and it seems pretty reasonable compared to others I've seen. So, if I read you right, the procedure would be as follows, if I also replace the rubber hoses:

Connect bleeder to MC
Pressurize to about 10 lbs.
Bleed one rear and one front corner to remove fluid from MC
Replace the hoses
Install the MC
Fill it up with fluid
Pour fluid in the bleeder and connect to the MC
Pressurize to about 10 lbs.
Bleed all corners one at a time
Top off the MC as required between bleedings
Job complete.

I like the idea of not having such a mess, esp. on the bench. Does this method of using the bleeder also get rid of the air in the MC? I was thinking about jacking up the rear end some to see if I could get the MC level, but maybe that isn't necessary.

One last question-- I've also read that you shouldn't reuse the fluid that you bleed off because it can have microscopic air bubbles in it which can cause problems later. You ever heard of that? I've reused it before (poured it back into the MC after bleeding) and never had a problem, but you learn things everyday. But are they always meaningful things?

[speedy72t]

brake fluid is cheap. don't reuse it. brake fluid absorbs moisture which lowers the boiling point and leads to rust in the system. Brake systems, especially with ABS, should be completely flushed with new fluid every two years at the minimum. That's actually right in the manual of many cars but it gets ignored.

There is no reason to use the pressure bleeder to empty the system. Just open up the bleeders and it should just gravity bleed right out. You can often do the same thing when you put in the new one. open up the bleeder and fill the master cylinder up. When fluid starts running out close the bleeders. Then you can use the pressure bleeder to pressurize any air out.

A friend of mine is a pretty serious mechanic and he hasn't used any other method than just a gravity bleed in his entire career. This includes cars he prepares for racing. I think he's nuts but it works for him. Just opens the bleeder and lets it run until nice clean fluid comes out. Closes it up and done. that's it.