Drum Brakes - when to arc shoes?

[Tom Drew]

OK. But that doesn't mean that the master cylinder was rebuilt properly OR is functioning properly. I would remove, inspect, and confirm proper operation if you haven' done that.

[ScooterABC]

Originally Posted by Tom Drew

OK. But that doesn't mean that the master cylinder was rebuilt properly OR is functioning properly. I would remove, inspect, and confirm proper operation if you haven' done that.

Thanks, I'm not close to the point of removing the master cylinder, and if I did I don't know what I would be looking for. It isn't leaking. Fluid is not escaping around the piston (or I would lose pedal pressure at the point when I finally get pedal pressure). I would sooner remove the shoes and drums and have them arced and see what the status is. And replace the front hoses that I know to be too long.

If you have a better argument for why I should remove it prior to the above then I would like to know what it is, cuz I honestly don't see an argument for it BUT I don't know everything about brakes either.

What I want to check next is to see if with wheels in the air minimal movement on the brake pedal causes the wheels to stop moving when spun by hand. That would tell me if I'm getting brake action corresponding to moving the pedal, or if there is some dead spot.

My top suspicions are the brake shoes not matching the drums, the drums being out of round, and possibly bulging hoses that are too long.

[97 SL320]

Out of round drums would give you a pulsation when slowing. The long hoses are not part of the peddle problem unless they are actually expanding under pressure.

[ScooterABC]

Originally Posted by 97 SL320

Out of round drums would give you a pulsation when slowing. The long hoses are not part of the peddle problem unless they are actually expanding under pressure.

Car elevated, I spin the front wheel. I here scratch scratch scratch as I spin the wheel, so something isn't quite round, but it's probably close enough. It seems impossible to adjust them so that the sound is uniform - all scratch or no scratch. On the front hoses, I agree and I need to feel them or hold them while pressure is being applied and see if anything is apparent. It's quite possible that I have multiple things wrong.

[FreddMersaid]

I didn't bother to read all the replies.

1. Is there any adjustment available on the Pedal push rod ?
1b. Is the Master cylinder bottoming out ? Check for the piston being pushed pasted the by pass point.

2. You may want to remove 1 drum at a time, then have a helper gently and slowly push down on the pedal @ your direction while you watch for movement of the shoes that indicates the cylinders are not frozen. Watching for shoe travel.

I have had frozen cylinders before or limited travel. If so, you may have to disassemble the cyl's and inspect them.

3. Could be a clogged or pinched brake line issue.

[ScooterABC]

Originally Posted by FreddMersaid

1. Is there any adjustment available on the Pedal push rod ?
1b. Is the Master cylinder bottoming out ? Check for the piston being pushed pasted the by pass point.

1. Yes, adjusted correctly. 1b. How would I tell? I mean what am I looking for on the piston of the master cylinder?

Originally Posted by FreddMersaid

2. You may want to remove 1 drum at a time, then have a helper gently and slowly push down on the pedal @ your direction while you watch for movement of the shoes that indicates the cylinders are not frozen. Watching for shoe travel.

I have had frozen cylinders before or limited travel. If so, you may have to disassemble the cyl's and inspect them.

Good idea on watching to make sure all cylinders are working. They are all new/rebuilt, but worth checking.

Originally Posted by FreddMersaid

3. Could be a clogged or pinched brake line issue.

Lines are new so clogged is unlikely. Lines are too long and I don't think they are pinching but I will check again.

Thanks to you and everyone helping me with this. I'm left with not understanding the problem and thinking that arcing the shoes is needed but it may or may not resolve the pedal travel issue.

[97 SL320]

Originally Posted by ScooterABC

Car elevated, I spin the front wheel. I here scratch scratch scratch as I spin the wheel, so something isn't quite round, but it's probably close enough.

Drums / rotors are never completely flat / round so a slight scratch is normal. It is only an issue if you get a pulse during braking.

[ScooterABC]

Originally Posted by 97 SL320

Drums / rotors are never completely flat / round so a slight scratch is normal. It is only an issue if you get a pulse during braking.

Thanks. No pulse during braking.

[FreddMersaid]

If you feel the push rod is adjusted correctly, then take off one line at a time at each wheel. Put the end of the line in a empty soda bottle. Fill the master resovior. Have a helper push down on the pedal. Watch the force of the fluid and mark the volume of one pedal push. Do this to each wheel. Start with an empty bottle each wheel.

If each wheel is not consistent, back track.

BTW, is there a proportionating valve in this system by any chance ?.
If so, you may have to start before that point. It should be just after the M.C. some where.

A rebuilt M.C. is my first suspect. But it could be ok if you prove it as I just explained.

[ScooterABC]

First of all, thanks for the reply. Questions:

Originally Posted by FreddMersaid

If you feel the push rod is adjusted correctly, then take off one line at a time at each wheel. Put the end of the line in a empty soda bottle. Fill the master resovior. Have a helper push down on the pedal. Watch the force of the fluid and mark the volume of one pedal push. Do this to each wheel. Start with an empty bottle each wheel.

That can't be done for all of the cylinders. The front brakes have two cylinders that are connected to each other by tubing. All I can do is verify that when you bleed the brakes a reasonable but not excessive amount comes out for each wheel. (not each wheel cylinder - one could be blocked, the other fine for a given front wheel). Given that, I'm not sure how removing a line (i.e., need to rebleed the brakes) can be done easily and the same thing is accomplished with a clear brake bleeding tube going into an empty bottle. I don't think one pump is going to be measurable accurately for volume in a soda bottle - it isn't that much - but I think the idea is that a reasonable pedal stroke of fluid ought to come out of the tube consistent with the speed of pushing down the pedal.

Originally Posted by FreddMersaid

BTW, is there a proportionating valve in this system by any chance ?.
If so, you may have to start before that point. It should be just after the M.C. some where.

A rebuilt M.C. is my first suspect. But it could be ok if you prove it as I just explained.

I do not see a proportioning valve. I'm not exactly sure what I'm trying to find out in this process. What I know from bleeding the brakes is:
1) if the bleed valve is closed there is resistance on the pedal, if the bleed valve is open there is only the resistance from the pedal spring.
2) if the bleed valve is barely open, I can push the pedal down with more resistance than if open and less resistance than if closed.
3. each pedal stroke in bleeding the brakes results in a squirt of fluid (although it's below fluid depth in the catch container) and the squirt of fluid does not have a pause as if there is a pinched hose. I can feel the fluid being expelled with each pump.

Given this, what could the master cylinder be doing wrong? I think what I need to check is to see if I get a hydraulic reaction at each wheel cylinder at the start of the stroke on the master cylinder (and consistent throughout the stroke). Does that seem like a good thing to do? Something tells me I should not push the brake pedal all the way down with no brake drums or the wheel cylinders might shoot out the ends. Not sure on that. If the brake drum isn't there to stop it, what does stop it before it pops out of the wheel cylinders...?

[97 SL320]

You really need to take the drums off for a look at the shoes / wheel cylinders. Looking at the front grease cap, it looks like it unscrews as per manual. I'd be using some sort of socket, using a pipe wrench might distort the cap while turning making it difficult to remove though is still might be possible.

Be sure to check lining condition and if the wheel cylinders are stuck or if what ever connects the wheel cylinder piston to the brake shoe is missing / short? .

[ScooterABC]

Originally Posted by 97 SL320

You really need to take the drums off for a look at the shoes / wheel cylinders. Looking at the front grease cap, it looks like it unscrews as per manual. I'd be using some sort of socket, using a pipe wrench might distort the cap while turning making it difficult to remove though is still might be possible.

The problem is that the strange cap has a hex head with rounded corners. Unless it were a 6 point socket there is no way it would work. I suspect I'm going to need to make a new tool, perhaps out of wood.

[FreddMersaid]

I didn't know you have dual Slave Cyl's on the front.

My questions now are:

1. Are the front cylinders in series ? (Meaning fluid goes though the first then the second and you bleed both from the second. ) ?

2. Have you had the drums off yet ? (I saw the photo of the front drum you posted, if the center cap is threaded it can be measured with a tape measure if nothing else, Or a caliber would be best, then get the right size 6 point socket)

3. Post some photos of the front brakes with the drums off.

4. Have you adjusted the shoes with the wheels off the ground, adjusting until there is some drag on the drum then backing the shoes off just a bit ?

5. Are you starting your bleed from the wheel farthest from the M.C, then next farthest etc ?

6. How are the rears working ?

[ScooterABC]

Originally Posted by FreddMersaid

I didn't know you have dual Slave Cyl's on the front.

My questions now are:

1. Are the front cylinders in series ? (Meaning fluid goes though the first then the second and you bleed both from the second. ) ?

Yes.

Originally Posted by FreddMersaid

2. Have you had the drums off yet ? (I saw the photo of the front drum you posted, if the center cap is threaded it can be measured with a tape measure if nothing else, Or a caliber would be best, then get the right size 6 point socket)

I have only had the rear drums off. I think I'm going to have to make a tool to take off the front hub. I don't think it's possible to get a large enough 6 point socket, seems like those are always 12 point. Am I wrong on this? I have a caliper to measure the size.

Originally Posted by FreddMersaid

3. Post some photos of the front brakes with the drums off.

I'll take lots of pictures when I take the drums off, hopefully this weekend.

Originally Posted by FreddMersaid

4. Have you adjusted the shoes with the wheels off the ground, adjusting until there is some drag on the drum then backing the shoes off just a bit ?

Yes. I don't know of another way. Certainly isn't going to work with the wheels on the ground.

Originally Posted by FreddMersaid

5. Are you starting your bleed from the wheel farthest from the M.C, then next farthest etc ?

Yes. It should be noted that the brakes were said to be bled before sale and when I bled them I concur that they had already been bled. No air bubbles anywhere.

Originally Posted by FreddMersaid

6. How are the rears working ?

Hard to say. Certainly not well. Other than being able to lock the front left wheel when braking hard, I have no sense for which other brakes are applying pressure and what time. FWIW the emergency brake works fine.

[97 SL320]

Originally Posted by ScooterABC

The problem is that the strange cap has a hex head with rounded corners. Unless it were a 6 point socket there is no way it would work. I suspect I'm going to need to make a new tool, perhaps out of wood.

Some 4 wd trucks from the 70's 80's + used rounded corner sockets for the front bearing nuts.

The cap should not be very tight, try a large crescent / pipe wrench / channel locks.