Brakes lost after rebuilding/cleaning calipers, 85 300TD - PeachParts Mercedes-Benz Forum

tomas_maly · #1 02-28-2012, 07:54 AM

I noticed that my front calipers on my 85 300TD were sticking and so I've taken them apart, removed the pistons, cleaned them off with brake parts cleaner, and put them back on.

I've been using a motive power brake bleeder to pressurize the master cylinder reservoir (after topping the fluid off), and then opening the bleed screw until it fills to approximately 60-80cc's.

I don't seem to be getting any air in the brakes any more, but still, the brake pedal goes entirely to the floor without any resistance. While doing this, I see that the caliper does not lock onto the rotor until perhaps 4-5 full slow pumps on the brake. And once the brake is engaged, letting go of the pedal keeps it stuck there, ie it does not let go of the rotor.

I'm thinking either there is air in the brakes, or that the calipers simply need to be replaced. The pistons seemed to have rust on them and were a pain to get off, so I'm thinking maybe even with them clean, there is some sort of rust/corrosion/etc inside of the caliper itself, between where the brake line screws on and where the fluid comes out for the pistons? I can try to use the brake cleaner to spray through the holes and see if that helps.

I have noticed that on the driver's side caliper, about every other purge, there is a sudden burst of air that comes shooting out of the bleed screw along with the fluid - and it doesn't seem to stop, which makes me feel like I've drained the master cylinder completely, but when I depressurize the power bleeder and look in the reservoir, there clearly is fluid in all chambers. I fill the fluid back up and start again, and suddenly no more air comes out.

Am I right in the assumption that even if the reservoir is completely drained, that I should be able to use the power bleeder to pressurize fluid through the brake lines and eliminate any air? (Though I really am trying to not empty it, I usually stop between 40-60cc's each before refilling) Perhaps it may take several fluid purges per caliper?

mespe · #2 02-28-2012, 08:49 AM

If there's one thing I've learned about brakes, it's that you can never bleed them too much. Unless of course you empty the reserviour, then you got to start over,,,

tomas_maly · #3 02-28-2012, 09:08 AM

I'm assuming that if there is air in the system, as long as the brake fluid is clean/fresh, then I can just re-use it instead of going through 2-3 quarts of brake fluid in this whole process. At this point, there is no more 'old' fluid in the system and the fluid that comes out is clear/pure. It just doesn't make sense to spend $30 on brake fluid when 2/3rds are just disposed anyway.

Diesel911 · #4 02-28-2012, 11:08 AM

I am wondering why you did not install a new Caliper Seal kit into the Calipers.

Have you been bleeding the Brakes in the right order; starting with the Caliper that is the greatest distance from the Master Cylinder?

Did Fluid come out of all of the Brake Bleed Screws?

It could simply be coincidental that after flushing out the crud inside of the Master Cylinder it is now not working.

tomas_maly · #5 02-28-2012, 11:36 AM

I didn't install a new seal kit because 1) it wasn't leaking in the first place and 2) I didn't have access to new ones. Since I've decided to slow down with the repair (using my other car in the time being), I'll take the time to order them online. The dust boots were torn apart and I noticed they have a metal ring on them which may help seat the pistons properly from pushing out slightly lopsided.

vstech · #6 02-28-2012, 09:43 PM

you have air in your master cylinder.

I'd get a bench bleed kit, and pump and pump and pump until there are no more bubbles.

please describe your bleed practice, step by step. I don't understand your "until it fills to approximately 60-80cc's" part.

79Mercy · #7 02-28-2012, 09:49 PM

Did the master cylinder go empty?

If not im thinking the caliper seals are ruined, you should never take them apart without replacing the seals in them. Replace the calipers?

tomas_maly · #8 02-29-2012, 09:33 AM

I have new caliper seals and boots on order. I'll wait to finish the job and just reseal all the calipers (since they all seem to be stuck). The car was purchased 9 months ago and was a real neglected thing, essentially a lemon, sitting for probably a few years in someone's yard. The brakes were never re-done.

As per my bleeding technique (I would appreciate advice as I seem to have mixed results):

The car is level on the ground and I can reach the bleed screws without lifting the vehicle up at all.

Starting with the furthest caliper (passenger rear):

1) Fill brake fluid in both reservoir chambers up to the 'max' line.
2) Attach the motive bleeder to the reservoir and pump it up to 15-25 psi. (maybe the exact PSI matters?)
3) Attach a clear plastic hose (came with my mityvac kit) to the bleed screw. The hose goes into a small container (again, from my mityvac kit).
4) Turn the bleed screw open (clockwise), probably 1/2 turn or so.
5) Let the fluid fill to 40-66cc's (the container is marked so I know) and then close the bleed screw. If I let it go any more, then there's a chance the cylinder will bleed dry depending on which caliper I work on (as the rear brakes seem to have a smaller reservoir chamber than the front brakes).
6) De-pressurize the power bleeder (by slowly unscrewing it), and then fill new fluid back up to the MAX line
7) Pressurize it again, and then bleed out another 40-50cc's as per above. Usually by this time there doesn't seem to be any bubbles anymore.
8) Repeat for the driver rear, passenger front, and then driver front brakes.

Is there something fundamentally incorrect about this process? Should I keep on doing it for 200cc's or so (instead of 80-100cc's) per brake caliper to make sure the air gets out? I believe the FSM says 80cc's is sufficient, but I'm not sure what may be necessary when there's air in the system.

I'm assuming a "bench bleed kit" functions the reverse of the motive power bleeder, by sucking out fluid from the caliper end, and that there is some tight fitting for where the bleed screw goes? Is there much difference in performance? I would think the concept of pressurizing a full reservoir and letting it flow out would be sufficient. It's rather quick.

I also had the same nightmare of a situation on my 300SD vehicle. Air was stuck somewhere and it wasn't until I got the Motive bleeder that I was able to fix it. Before I had to press the pedal 1/2 way before I got any response. Now the brakes are incredibly sensitive and work beautifully. So I'm fairly confident that the Motive bleeder is sufficient in a worst-case scenario.

It seems that all of the calipers are stuck, so I may need to rebuild all of them at once before I try to bleed anything. Is it possible that air in one (or more) calipers could be causing other calipers (or the brake pedal itself) to not function optimally? The pedal doesn't get stuck, it just seems to go to the floor with barely any resistance.

vstech · #9 02-29-2012, 10:28 PM

no, the bench bleed kit is for the master cylinder only. any auto parts store will have the bench bleed kit.

you remove the brake lines, and install plastic fittings and clear tubing, and you fill the reservoir, then pump the MC until all bubbles are gone from the clear tubing.
you are just pumping the fluid out of the mc and back into the reservoir.
once that is done, you then reconnect the brake lines, and use the motive bleeder to clear all air from the lines and the calipers.

compress ignite · #10 03-01-2012, 03:32 AM

DO Not,Do Not,Do Not EVER reuse Brake Fluid!
[AND once a Brake Fluid container is open LeftOver's are JUNK.]
(No Matter what She Promises You!)

myskyon · #11 03-01-2012, 10:31 AM

i also have the motive power bleeder and i have never had any problems bleeding the brakes on any car.

1 get 1qt of dot4 brake fluid and put it all in the power bleeder.. then hook up to master cyl
2 pump pressure up to 10psi and then go the passenger rear and bleed it untill you see no more air bubbles, no ammount to bleed out just bleed till the air stops coming out
then move to the divers rear then pass front, then drivers front.
when you are done bleeding position the motive so that the pickup tube is no longer in brake fluid then open the driver side bleed screw, and watch the fluid in the hoses to reach the master cyl then close the bleeder and you are done...

you should never open the power bleeder in the middle of the job it has the possibility to screw every thing up and introduce a lot of air in the system...
ask me how i found that out... lol it should take no more than 30 min to flush a brake system...

charmalu · #12 03-01-2012, 11:34 AM

(QUOTE) I have noticed that on the driver's side caliper, about every other purge, there is a sudden burst of air that comes shooting out of the bleed screw along with the fluid - and it doesn't seem to stop, which makes me feel like I've drained the master cylinder completely, but when I depressurize the power bleeder and look in the reservoir, there clearly is fluid in all chambers. I fill the fluid back up and start again, and suddenly no more air comes out.

Am I right in the assumption that even if the reservoir is completely drained, that I should be able to use the power bleeder to pressurize fluid through the brake lines and eliminate any air? (Though I really am trying to not empty it, I usually stop between 40-60cc's each before refilling) Perhaps it may take several fluid purges per caliper?

This sounds to me, you are using your Motive Power Bleeder to pressure your Master Cylinder with just air, and not filling it with Brake Fluid to purge the system. am I right?

You fill the motive with Brake Fluid, attach the cap supplied with the Motive to the Master Cylinder reservor. pump the Motive up, and then bleed each bleeder starting from the right rear.

Also reading through what you post here (In the below quote) also comfirms with me you are not filling the Motive Power Bleeder container with fluid, and just filling the MC reservor and purging with air pressure from the motive.

(QUOTE)
1) Fill brake fluid in both reservoir chambers up to the 'max' line.
2) Attach the motive bleeder to the reservoir and pump it up to 15-25 psi. (maybe the exact PSI matters?)
3) Attach a clear plastic hose (came with my mityvac kit) to the bleed screw. The hose goes into a small container (again, from my mityvac kit).
4) Turn the bleed screw open (clockwise), probably 1/2 turn or so.
5) Let the fluid fill to 40-66cc's (the container is marked so I know) and then close the bleed screw. If I let it go any more, then there's a chance the cylinder will bleed dry depending on which caliper I work on (as the rear brakes seem to have a smaller reservoir chamber than the front brakes).
6) De-pressurize the power bleeder (by slowly unscrewing it), and then fill new fluid back up to the MAX line
7) Pressurize it again, and then bleed out another 40-50cc's as per above. Usually by this time there doesn't seem to be any bubbles anymore.
8) Repeat for the driver rear, passenger front, and then driver front brakes.

Charlie