Single Metallic "Clunk" as Brakes Applied: then More Braking Occurs

[Cardinal Grammeter]

I've changed this title taking Wheel Cylinder out of it BECAUSE if a WC was sticking, the truck would be pulling and pulling more after the clunk.

 

This is what happens:

 

Getting some word meanings down:  Applying Force to pedal produces Braking.  More Force, More Braking.

 

This is what happens:

 

Everything seems to work just fine, during Braking truck doesn't pull to the side - it goes straight.

 

Over time, seems like less Braking per pedal Force.

Then, when applying a relatively high pedal Force, there is a loud clunk, and Braking increases substantially.

From then on it's like I have a more powerful brake booster getting more Braking for a given Force.

Over time, seems like less Braking per pedal Force.

etc., etc.

 

I originally thought it was a worn solid inner control arm bushings shifting - especially since at least one did not want to take grease (indicating it was perhaps worn and sloppy and full of rust/metal debris.)  But then it became clear Braking was increasing. 

EDIT NOTE:  It could be a silent hydraulic issue but the instant change in vehicle breaking could cause the worn ICAB to shift and clunk. 

 

Sticking with hydraulic causes:

 

*) NOT Wheel Cylinder since is symmetric issue

*) Master Cylinder?

*) Brake Booster?

*) NOT Proportioning Valve since there isn't one.

 

Thanks

Tom

Edited October 6, 2020 by Cardinal Grammeter

[Cardinal Grammeter]

UPDATE:  (6 months later)

 

New shoes on front.

Made sure the sliding star adjusters were free - one side reasonably free, other side a bit stiff but both improved with Dry Lube w/Teflon spray.

Much clunking eliminated, remaining clunking less effect on braking.

 

NEW THEORY:   STUCK/STIFF SLIDING STAR ADJUSTERS

 

If SSA does not slide under light braking, the wheel cylinder will apply force to ONLY 1 SHOE.  In this case, there will be a negligible pull to the free side that will be unnoticed since only light braking.  When the clunk occurs, THEN both shoes are now working, braking is increase, and here is the key point:  now both sides are braking correctly and there is no pull.

 

The pull occurred BEFORE the Clunk.  NOT AFTER IT.

 

Issue is the thin piece of sheet metal that is spot welded to the backing plate where the slot goes through.  Since a spot weld, we have sandwiched metal which wicks moisture and generates rust scale which grows, spreading the pieces making the SSA hard to slide.

 

My approach was to use my needle descaler as rigorously as possible to break up and expel that "sandwich" rust.  Unfortunately it is not 100% effective and I really avoid those awful brass "fork" shims.  NOTE TO SELF:  Next time I see them FS, I should buy some. 

 

 

 

[datzenmike]

The adjuster should center itself when brakes are used and remain there, moving only when there is enough wear for an adjustment to be made. The adjuster has 3? brass C clips that interlock and hold it but allow it to slide. (with effort) Your 620 should have a rubber cover that clips over these C clips to keep moisture and dirt out. Are yours still on there?

[Cardinal Grammeter]

Yes, all is intact but only 2 brass clips - I think it is 2 clips and a shim sized to fit?  That rust sandwich issue makes them tight with just the 2 clips.

 

I've got new rubber boots but the perimeter lip on the backing plate is rusted to the point where it's amazing they stay on.  It's mosly rust in the groove I think.  I should take the oxyacetylene torch and descale them.  NOTE:  Unfortunately I won't be addressing these issues because there are so many other things I have to attend to and I still have to find some time to PUT MY 5 SPEED IN!  (will have to make a decision about the clutch)

 

The adjuster has to move back when you let off the brake, other wise it would not allow the "driven" shoe to retract from the drum.  (I'm calling the shoes "driving" and "driven" - the "driving" shoe the one that the wheel cylinder piston "drives".)   NOTE:  Is the Primary the one that the wheel cylinder piston pushes on?  ...I'm thinking yes.  It's the front shoe.

 

 

Edited March 13, 2021 by Cardinal Grammeter

[datzenmike]

Each wheel cylinder has two pistons, one for each shoe. Both shoes move apart to contact the drum and the return springs pull them back when released.

 

There are leading and railing shoes. A leading shoe is pushed out in the direction of drum rotation and actually gains a mechanical advantage over the trailing shoe. Often the leading shoe friction material is mounted closer to the wheel cylinder and the trailing shoe farther away to maximize and take advantage of this servo effect and reduce one shoe doing more work and wearing out sooner. If both sides leading and trailing shoes are mistakenly reversed the vehicle will brake more effectively backing up than going forward. If one side reversed the vehicle will pull to one side.

 

Some shoe makers have do distinction between leading and trailing shoes and the friction material is bonded in the middle of the shoe.

 

 

 

Adjuster at top, wheel cylinder at bottom.  Forward motion counter clockwise and to the left. These are very worn but thumb and finger are on edge of lining and wheel cylinder in between. The right side is much closer to the cylinder

[Cardinal Grammeter]

my 74 620 has one-sided wheel cylinders - there is just one piston.  Where the other piston would be, that is a solid anchor to the backing plate.

 

It is because of that, that the adjuster has to slide to unload the shoe that is being driven into the anchor.

 

Also there are no hold down springs and both shoes are identical.

Edited March 14, 2021 by Cardinal Grammeter

[datzenmike]

 I see that these type are only on the fronts. I had a '78 which had disc and assumed the rears and front were the same.

 

 

There should be two horizontal return springs one above and below the axle pulling the two shoes inwards.

 

As I said some shoe makers don't distinguish leading and trailing shoes, but is it possible that you might have both leading and both trailing on the same sides?

 

 

 

I guess this single piston works but how does the shoe without the piston ever wear at the tops and this set up means every time the brake is applied the adjuster must slide.... must be a hell of a lot of were on it.

[Stoffregen Motorsports]

20 hours ago, datzenmike said:

Each wheel cylinder has two pistons, one for each shoe. Both shoes move apart to contact the drum and the return springs pull them back when released.

 

There are leading and railing shoes. A leading shoe is pushed out in the direction of drum rotation and actually gains a mechanical advantage over the trailing shoe. Often the leading shoe friction material is mounted closer to the wheel cylinder and the trailing shoe farther away to maximize and take advantage of this servo effect and reduce one shoe doing more work and wearing out sooner. If both sides leading and trailing shoes are mistakenly reversed the vehicle will brake more effectively backing up than going forward. If one side reversed the vehicle will pull to one side.

 

Some shoe makers have do distinction between leading and trailing shoes and the friction material is bonded in the middle of the shoe.

 

 

 

Adjuster at top, wheel cylinder at bottom.  Forward motion counter clockwise and to the left. These are very worn but thumb and finger are on edge of lining and wheel cylinder in between. The right side is much closer to the cylinder

If the brake shoes are installed with the wrong shoe leading, it is possible for it to grab rather quickly. We found this out the hard way on a friend's 620 many years ago. No shop could diagnose the problem, but once we swapped the shoes, the problem went away.

[Cardinal Grammeter]

I carefully looked at the new shoes and all 4 looked the same.  I did note on one that there was some white ink lettering on it and a "FF" which means a good high coefficient of friction lining.

 

It did not occur to me to see if the ink was the same on all 4 shoes.

 

I will say with the new shoes and "Tefloned" sliders, this clunking is definitely reduced.  For years, it clunked and I thought it was a rusted/loose threaded solid bushing upper inner control arm bushing shifting.  Only much later did the abrupt change in braking force (at constant pedal force) start happening with the clunks.

 

And YES, I remember the rears DO have TWO piston wheel cylinders. Because I was using a ball bearing and c-clamp to free up a stuck one.  (Used ball bearing to push piston in, then clamp and push out with brake pedal.

 

I don't have a lift and I hate creepers so it is with GREAT reservation that I go under since I'm also lazy and TOTALLY HATE setting jack stands. 

 

(Someone should make a jack stand system where you can wheel them under the car with a long handle, position them looking through cheap Chinese video camera, and then raise them somehow through handle.  They key would be that you never have to get on your hands and knees and put your ear to pavement to set the jacks.  LOL)  NOTE:  I also would like to get a WalMart scooter and see if I could turn it into a motorized creeper.  With baskets at my side for tools, joy riding around under the car would be cool.