D25 Brake problem... sporadic lack of clearance shoes-to-drums

[bamfordsgarage]

I recently rebuilt the factory brake system on my Canadian 1947 D25 and am experiencing an occasional "snugging up" of the shoes into the drums whilst driving. Perhaps a Forum member has also had this problem and/or can shed light on the probable cause...

Work done: Relined all brake shoes (3/16" lining), new wheel and master cylinders (Raybestos from Rock Auto), replaced four brake lines (to rear, on rear axle, and within the front wheels), bled & flushed the system, and manually adjusted the new shoes to minimal clearance to drums (such that when rotating the drums by hand, each rev would would drag just the tiniest bit at one spot, presumably the result of a not-quite-perfectly-round drum). The drum surfaces were reasonably good with no significant ridges or grooves and generally smooth contact area.

I noticed some metal filings inside the master cylinder reservoir when unpacking same from the box — took it all all apart and flushed it thoroughly with BrakeKleen and then brake fluid. I left a small ceramic magnet in the reservoir to catch any stay residual metal bits. The magnet is covered in a fine black fuzz.

Observations: At random intervals (for example: one day, one week, 20 blocks, 2 days, etc.) I can sense the brakes not releasing fully but certainly not locking on hard. The kind of thing where one can hear a bit of a drag when moving very slowly, and the car will not coast freely to a stop but stops a little sooner than it should. The problem does not get worse than this, but does not correct itself either.

Otherwise, the brakes perform well, stopping the car smoothly and quickly, with no drag to either side. There are no apparent hydraulic leaks. Pedal travel once past free play feels too short — with all my not-inconsiderable effort on the pedal, it still stops 2+" short of the floorboards.

My quick-fix when this dragging occurs is to crawl under and crack open one of the bleeders for a fraction of a second. A bita fluid spurts out and we're good to go another 20 blocks or two days or whatever. I have done this procedure on the RF, LF and LR wheels, and it worked each time.

Checked: Pedal free play at the linkage is OK. There is no pedal or linkage pressure forcing the master cylinder piston off the piston stop until the pedal has moved an inch or so.

Peering into the master cylinder whilst pushing on the brake pedal does show a flow out of (up through) the compensating port, but not necessarily with the volume/force I would expect.

However, I cannot poke a small 0.021"dia probe down through the compensating port —into the port, yes, but not through. The probe is stopped by what is likely the master piston primary cup.

When the probe is resting in the port hole (and, presumably, on the side of the primary cup), a forceful push on the pedal spurts out fluid through the port and the probe is dislodged.

Speculations: Shoes too close to drum? I think not, in that my quick fix of bleeding off residual pressure corrects the symptom — if the shoes were adjusted too tight, relieving the pressure would not affect this (I think).

Poor/broken/weak brake shoe return spring? Perhaps, but why would the problem be so sporadic?

Residual Pressure valve faulty/too strong? Again, maybe, but would this not be a problem continuously?

Piston not retracting far enough when pedal is released? This would explain why I cannot push my probe through the compensating port. From what I've read, the master cylinder piston should retract far enough that the primary cup fully clears the compensating port. However, piston retraction does not seem to be an adjustable distance and the piston does appear to seat firmly against the stop.

Assuming I assembled the master cylinder properly after flushing, then the retraction distance is factory set. If it is not correct, then other people using this same part# would have experienced the same issue (and, maybe they have...)

Possible fixes: Remove, examine and reassemble/reinstall master cylinder in case I put it together wrong the first time.

If no improvement, fabricate a gasketed spacer to move the piston stop about, say, 1/8" further away from the cylinder casting, to allow another 1/8" of piston travel and thus clear the compensating port.

Or chuck the cylinder in the lathe and take off, say, 1/8" from the backside to allow it to come closer to the piston stop, again to clear the compensating port.

However, unless my situation is a known problem with this master cylinder, I am loath to start modifying parts to alleviate a problem that probably originated elsewhere.

You would think from looking at my shop that I never throw anything away, but I did unfortunately haul the master cylinder to the scrap yard a while ago along with some other junk — thus I cannot examine the old to see what it could tell me about the new.

_ _ _ _ _ _ _ _

If anyone is still reading after all this longwindedness, I would very much appreciate any comments or suggestions about this issue and what might be the cause. Thanks!

[Niel Hoback]

Is it possible that the mc piston is sticking in the bore near the end of it's travel? If you have the pushrod adjusted so its not touching the piston, then something is keeping the piston from fully returning. You could take the piston out without removing the mc just to examine it.

[desoto1939]

On my 39 Desoto the MC has a proportioning value at the end of the body of the reservour. You might have the value installed backwards. This allow the fluid to retract. Also check that the two releif holes in the MC are open. There should be two 1 behind the piston and one after the pist.

I had a factory installed original MC and had a problem. When I tookthe MC out of the car I founf that one of the weep holes was not completely tapped open. Sticke a small wire inthe holes to see if they are clear. You might have to retap one of these holes.

Hope this helps.

rich hartung

[bamfordsgarage]

Thanks both for your suggestions.

Neil, I agree there might be something keeping the piston from retracting fully (other than the dedicated piston stop bolted onto the end of the cylinder). That would explain this weird behavour.

Now that you mention it, this could be checked without actually removing the cylinder and that sounds like it should be my next step.

Rich, I do know that the compensator port (in front of the piston) is blocked although it appears to be blocked only by the piston primary cup (the piston having not fully retracted). I will double-check this when I followup on Neil's suggestion.

[greg g]

could also be as simple as a bit of debris inthe relief hole in the MC. this lets fluid and pressure bleed back into the reservoir when the pedal is released. Check that both hole in the bottom of the MC are clear.

[Young Ed]

Did you replace the flex lines?

[desoto1939]

You stated that you put a magnetic inthe MC to catch the fine metal parts. There should not have been any metal inthe res. There still might be some metal inthe MC that is clogging the two relief holes. Clean the mag and then insert to catch any metal

rich Hartung

[Jerry Roberts]

Did you replace the flex lines?

As Young Ed mentioned , the flex lines might be the problem . Sometimes they partially collapse inside and restrict the flow of fluid back to the master cylinder , this would explain your dragging brakes .

[james49ply]

did you check the pedal itself? maybe some of the pivot points are sticking and not allowing the full return? Just a thought,

[bamfordsgarage]

Hello all, thanks for the additional comments.

Yes, the flex lines were all three replaced, just forgot to mention it in my original post.

Out for dinner now with my beloved then will open up the master cylinder for a look (obstructions, ports) later tonight or first thing AM.

[55 Fargo]

I think your MC is suspect, even though it is new from Rock auto, it could be flawed somehow.

I lean toward having old MCs sleeved, and rebuilt.

I do know 2 Guys in Winnipeg, that have NOS Mopar MCs, if you ever need one.

Good luck, you seemed to have covered most itmes in your brakes, myifrst thought was a couple of weak return springs.

Those new wheel cyls, are the push pins, the same length inrelation to the old ones, I have heard this can be a problem area on replacement new wheel cyls.......Fred

[bamfordsgarage]

I found the problem. Thanks to all who contributed.

Will post explanation, pix and The Big Question later this AM...

[Niel Hoback]

Ooooooo, the tension. C'mon, hurry up.

[bamfordsgarage]

Sorry Neil, I was in the workshop since five this morning and needed breakfast and a nap before posting.

_ _ _ _ _ _ _ _ _ _

Rockwood was right -- near as I can tell, there was a problem with the master cylinder. Having said that, I take equal credit/blame for assuming new repro parts are always OK right out of the box. If only I had ensured both ports were open before installing the dang thing...

Photo A: The bent wire is the length from the end of the master cylinder to the compensating (front) port. When the piston assembly is fully retracted, it must be past this point in order to clear the port. Not gonna happen.

Photo B: I believe this thick hard rubber washer between the piston and the secondary cup is the problem.

Photo C: The factory repair manual shows a skinny little metal washer #15 at this location (with axial holes through the ring). Note also the secondary cup shown at #16 has a hole in the centre.

Photo D: The secondary cup in my repro master cylinder is molded around a metal core and dished in the centre back toward the piston. I figure this one piece should replace the original factory setup of #15 and #16 and allow fluid to zip past the cup when the brake pedal is released/piston is retracting.

Photo E: Without the big fat washer, the piston assembly is short enough to clear the compensating port nicely when retracted.

_ _ _ _ _ _ _ _ _ _

I'm generally very careful to reassemble things exactly as they come apart -- its been over a month since I took the MC apart to flush out the machining debris and while I can't be positive, I am pretty sure it went back together exactly the way it came apart, with that fat washer between the piston and the cup. There's certainly no where else it would go in there.

I'm inclined to ditch the fat washer, button up the MC and put it back in the car.

Comments?

[Niel Hoback]

I don't know where that washer goes, but it doesn't go there. Chuck it. Put the MC back together, bleed the brakes and drive it.

[Dodgeb4ya]

That thick rubber washer goes at the back (rear end) of the master cylinder- Between the check valve and the rear M/cyl end cap! Was that a Rock Auto professional grade M/C or the cheaper china made cylinder? I've been there done that on a 40 Ford. The Chinese M/C was faulty-had to return it and get the professional grade US made cylinder.

Edited September 5, 2010 by Dodgeb4ya

[bamfordsgarage]

Hello Dodgeb4ya -- hmmmmmmmmmmmmm, that was an interesting post. Thank you.

Rock Auto did not offer a choice of professional or cheap that I noticed on line, however the brand on the box is Raybestos and the cylinder has Made in USA cast on the side.

This afternoon I put the cylinder together and back in the car. My observation during the test drive was that pedal travel has increased significantly (no other adjustments made except ditching the washer).

I've been puzzling over this development and your statement that the fat washer belongs between the check valve and the casting would absolutely explain my new low pedal condition -- with no washer back there, residual pressure would obviously leak between the check valve and the end cap, allowing the wheel cylinder pistons to retract further, thus requiring longer pedal travel to bring the shoes (back) into contact with the drum.

It is also a much more likely scenario that the fat washer didn't end up in my cylinder by some fluke, but belongs in there after all and was merely mis-assembled at the factory.

This new development also increases the odds that I had a brain fart when I originally disassembled the cylinder to clean out that machining debris and put it back together wrong at that time.

If anybody needs me, I'll be out in the shop...

[Niel Hoback]

That washer is not shown in the shop manual, nor have I ever seen one in a MC. There is none in mine. Not in the parts book either.

[bamfordsgarage]

I have the Canadian parts book and the reprint US shop manual, and yes, that washer is not shown in either.

But it makes sense to have something there -- without some sort of seal, surely the residual pressure would eventually leak past the valve/end cap interface (metal on metal). A tiny imperfection in the surfaces or littlest bit o' grit in between would be enough.

Sure wish I hadn't sent my original MC to the scrap yard!

Neil, might there be some sort of rubber seal or other sealing strategy built in to the end of your residual pressure valve?

Maybe someone else here has a spare MC handy that they could look at and add to the discussion?

[Dodgeb4ya]

That washer is not shown in the shop manual, nor have I ever seen one in a MC. There is none in mine. Not in the parts book either.

The reason the shop manual does not show that thick rubber washer is that the original check valve has that rubber washer(seal) built into it. I have one old OE MoPar check valve showing that it already has the rubber seal as part of the valve assembly which must seal against the M/C end cap to function correctly. Some pics of assembly order and a pic of an original used MoPar 1 piece check valve with the rubber seal built into it. You cannot buy them anymore.

Bob

[Andydodge]

Also for what its worth I'd ditch the magnet, the cylinder should be clean enough to not ever need a magnet, and if its been resleeved it should have been done with either a brass or stainless sleeve and therefore the magnet would be useless.......have never heard of a magnet in a master cylinder.......my 1.5 Oz cents..........andyd

[55 Fargo]

Andy, I believe the Bamford bought a new Raybestos MC, and did not have his old MC re-sleeved.

I lean toward, NOS mc, wheel cyls, with new rubber installed, than what is offered new these days. I think if and when I need to do this, I am getting what I have re-sleeved.

The stock brakes perform very well, when set-up correctly and are in good shape, DodgeB4ya, gave me some real expert advice on these brake systems, at this point don't seem to see the need for a disc brake conversion, for my driving needs.

Don C is going through a lot of trial and error on his MC conversion kit, not sure this happens in all cases, but he is certainly going through a lot to get his working correctly.......Fred

[bamfordsgarage]

Thanks Dodgeb4ya, for the details of the original and replacement seals at the back end of the MC, and for everyone who took an interest in my problem and contributed to the solution... this forum is a pretty great place.

End of the story: MC out and in again this morning, with the washer installed under the relief valve, brakes working dang near perfect. Right on.

[Dodgeb4ya]

Thanks Dodgeb4ya, for the details of the original and replacement seals at the back end of the MC, and for everyone who took an interest in my problem and contributed to the solution... this forum is a pretty great place.

End of the story: MC out and in again this morning, with the washer installed under the relief valve, brakes working dang near perfect. Right on.

Gotta love that!

Bob