240Z Rear Wheel Cylinders Failing Consistently!

[Nickymac]

Hi All,

 

First post here so please forgive any newbie mistakes. I shouldn't even be here, as the car I race in historics is actually an Elva Courier. The car has been racing in historic for 30 years, between ourselves, Brian Sampson and Winston Kim.

 

However, it's got 240Z drums at the rear and the wheel cylinders continue to fail. When I say fail, after about 20 laps (with new cylinders - or honed and rekitted ones) They start to weep fluid. That weep becomes a leak and then everything gets wet and messy. Every session means the drums come off, a can of brake clean gets used and if the circuit is hard on brakes, I may pull the cylinders off, hand hone them, put on some new piston seals and reassemble to get through the meeting.

 

I have been trying to solve the problem for 2 years and am gradually working through every possible problem without success.

 

So stop me now if you've had the problem and worked out what caused it!!

 

The backing plates are drilled, the drums have  large vent holes, there's good airflow, heat isn't the problem. It seems that the problem is mechanical. i say that because after just a few races, there's a very clear wear mark in the bore, that indicates the piston is cocking. Last weekend at wakefield park, (fresh hone and new rubbers) I pulled the pistons out and just replaced the seals only for them to leak in the next race immediately. So the bore must have been compromised.

 

We have tried new OEM cylinders, sleeved cylinders, 2 different types of seals that are the same size, measured the piston to bore clearance (it's correct), tried little O rings on the piston neck to stop the seal sliding up and down, we are running castrol 4.1 fluid, etc etc. The shoes are always adjusted up.

 

There are 2 avenues left I think. Have I been reassembling the drums with the wrong spring configuration? I've tried it a couple of ways, one where the you have to hold the shoes lightly against the backing plate, before securing them with those little spring retainer arrangements. The other way round the shoes really wanted to roll off the drum. Neither seemed to make a difference.

 

The other is the drums themselves which are at their max of 230mm. They've been on the car for 20 years. Whilst the linings appear to wearing evenly, it's possible the drum liner surface is no longer square and the linings are cocking under load. We are taking the whole assembly over to a couple of brake guys for their feedback, because after this weekend I've just about reached the end of ideas.

 

Any feedback or thoughts would be warmly welcomed!

 

Kind regards,

 

Nick 

[Agno]

Hi Nick,

 

Are you permitted to run the later type 260Z rear wheel cylinders in your class? I ask because the later (1976 onwards) 260Z came with a dual piston wheel cylinder arrangement which is infinitely better than the sliding piston arrangement found in earlier Z rear drums.

 

Without having checked it myself I would assume that the sliding single piston arrangement found on 240Zs would be prone to wearing one side of the cylinder wall (and also potentially cause uneven brake shoe wear) because as the wheel cylinder lever acts upon the piston and slides it up the bore, the leading brake shoe will be trying to pull the piston downwards and wearing one side of the cylinder prematurely. 

 

To convert to the later type you would need new shoes, wheel cylinders, adjuster mechanism and backing plates, the drums themselves are the same.

 

They look like this:

[260DET]

And perhaps the cylinder pistons are over extending.

[Agno]

Also out of general curiosity, Nick do you mind throwing up a photo of your drums and how they are drilled? There wouldn't be anything wrong with them I just want to see how others have drilled their drums so I can copy it!

[nizm0zed]

And perhaps the cylinder pistons are over extending.

This is where id be looking first.

Sounds to me like the piston over extends, looses its lateral support in the bore and cocks over.

Too much of that causes excessive wearing in one spot (localised wear mark) and the seals go from there.

[Nickymac]

Thanks all for responding thus far. We could probably go to a 260Z config but we raced this car ourselves back in the 80's when we originally restored it and never had a problem. The car has raced for 30 years without a problem. It's only the last few years that this has happened.

 

The over extending theory has merit - although they are adjusted up before every race. It's conceivable that they are expanding to such an extent that the piston has to travel right out. I did read of one driver who uses the handbrake adjuster mid race to wind the shoes out a bit to cope. It's conceivable that the linings we are using now have much more grip which is causing them to work too hard and get too hot and consequently extend too far. I created some extra cooling for this last meeting, but I think that was nullified by the fact that the circuit is so hard on brakes. 

 

Sandown in 6 weeks will be a better benchmark.

 

Thanks again for your thoughts, I sure would like to solve the problem.

 

Cheers

 

Nick

[dat2kman]

Me.

I pulled the handbrake on one/two clicks, to give a bit more pedal height.

 

Have you ensured the adjuster cone at top of backing plate has the tops of the linings snug to the drums?

I used "taxi" grade linings, hard, but stable, and would go out to 231 mm at this diameter, i would get odd "H" shaped cracks in the drum lining.

There used to be a popular "motorsport" lining, known as "Jolly Green Grabbers" for the greensh colour, when new, these worked very well. You'd need to find an old guy at an old brake re-lining shop, who may recall these!

 

Maybe try some fresh thicker linings, give them a chamfer on leading/trailing edges, but have the new linings correctly radiused to suit the drum ID, to ensure correct full contact.

 

I used to race in Group S, (aka, the Drum Brake Brigade!)

[Gtv240z]

I don't know much about the brake fluid you are using but the original system is designed for dot 3 fluid .

 

in many cases using dot 4 in a dot 3 system causes a lot of trouble . ruined master cylinders and brake pedal pressure drop are some of the symptoms.

 

some oil suppliers , even my mobil oil supplier tells me dot 4 supersedes dot 3 but this is simply not the case.

[260DET]

Did you drill your drums around the working surface Jason? I'd be very careful doing that as it may contribute to the drums expanding under heavy use.

[dat2kman]

Did you drill your drums around the working surface Jason? I'd be very careful doing that as it may contribute to the drums expanding under heavy use.

Have seen it done, with no adverse issues. That would help to "break" the gaseous bond, theory similar for the slotting of disc rotors.

I did mine on the front face, and chamfered the holes, to reduce possibility of stress risers. 3/4". x 6 holes,

[PeterAllen]

...

The other is the drums themselves which are at their max of 230mm. They've been on the car for 20 years. ..

 

I wondering if the drums have become 'soft' - annealed by repeated tempering under racing conditions, and thus expand at a greater rate towards the edge, creating a taper under operating conditions. The cooling holes may also have contributed if this is the case.

 

First check the bore isn't tapered. I wouldn't worry about going a little oversize to square the bore, but radius the shoes to suit. Give them a 20 lap test after machining.

 

Is the hub true and the mating face of the drum free of burrs?

 

WOW... Brian Sampson, now there's a name from the past. Cheetahs, etc.

Edited September 28, 2016 by PeterAllen

[Nickymac]

Thanks again guys for the feedback. Firstly there are cooling holes in the backing plate including a ring of 5mm holes right around the outer edge. The drum has 4 or 5 30mm holes but on the surface that faces the wheel. There are no holes on braking surface. 

 

The piston normally extends about one mm, how much further would it go when they expand? I can't imagine too far, but I might stick the GoPro down there for a session at Sandown and watch it!

 

Brake fluid is fine at Dot 4 , there are zero issues with compatability with 3. It's when you go to 5.1 that you get probs as that's a synthetic formula.

 

I don't think annealing is the problem, or we would have all heard of it happening with drums in the past. The taper of the lining face is the first thing to check I think as the piston is cocking somehow.

 

I am going to try and pull the handbrake on a few clicks, if piston protrusion is the issue then it may help.

But I keep coming back to the fact that the car has raced for30 years without this problem, what's changed?

 

 

Once again thanks for all your help.

Nick

[Scoota G]

As others mentioned get some pics of all the equipment you have. You may be using a combination of different Datsun parts causing the incompatibly. Pity you can't run discs on the rear as i have a new 300MM Rotor option for the 240Z 260Z  < (Shameless Plug) 

[Nickymac]

As others mentioned get some pics of all the equipment you have. You may be using a combination of different Datsun parts causing the incompatibly. Pity you can't run discs on the rear as i have a new 300MM Rotor option for the 240Z 260Z  < (Shameless Plug) 

They are absolutely standard 240z drum assemblies, the only exception being some cooling holes drilled. You can't fit 260Z/280Z cylinders, linings or drums to this backing plate. I will take some pics though, it's a good idea, somebody may well see something.

 

Cheers

 

Nick

[Nickymac]

If I had the backing plates swapped by mistake during the last resto,  the loads on the pistons would be reversed. The way it is now, the cylinder on the drivers side is on the left of the drum, thus the top lining is being pulled away from the piston as it actuates. Same load applies on the passenger side, although the piston is on the right obviously.

 

If the backing plates were swapped, the top lining would be forced into the piston as it actuates and perhaps affect its performance.

 

Just a thought.....

[Agno]

From memory the backing plates have L and R stamped on them to make sure that doesn't happen. Worth a look though I guess

[dat2kman]

Pretty easy to check if swapped backings, the handrake mech slo mst be to lower rear.

 

As above, check your top cone adjster, lining thicknesses,

F linings are lw, and too much slack on adjster, this can cause the piston in slave to over-extend

[Nickymac]

Pretty easy to check if swapped backings, the handrake mech slo mst be to lower rear.

 

As above, check your top cone adjster, lining thicknesses,

F linings are lw, and too much slack on adjster, this can cause the piston in slave to over-extend

Thanks mate, yes i did work that out the handbrake wouldn't be in the right spot! Linings though are thick and brakes are adjusted up before each session, linings lightly touching drums when you spin them.

 

What I have worked out is that I've swapped the cylinders over by mistake and the adjuster lever is now on the inside of the slave. The bigger linings spring and the lever are actually pushing against each other, perhaps this is contributing to the problem....

[dat2kman]

You solved it!

[HKSZ]

Jason what brakes or mods did you run up front to stay within historic group S?

[dat2kman]

Standard, factory fitted.

Rules allw holes for cooling, on rears.

To keep up the front, you will boil fluid, turn pads into apple crumble, and warp discs.

Add in there, severe radial micro cracks, and cracks to drum linings.

Treat them as consumables.

 

Funnily enough, when running in Sprints etc, against the massive bg braked 4 wheel dsced Z's runnng triples, bg mtrs etc,

The Group S repped Z's are as quick, if not quicker.

That is satsfying.

[Nickymac]

Standard, factory fitted.

Rules allw holes for cooling, on rears.

To keep up the front, you will boil fluid, turn pads into apple crumble, and warp discs.

Add in there, severe radial micro cracks, and cracks to drum linings.

Treat them as consumables.

 

Funnily enough, when running in Sprints etc, against the massive bg braked 4 wheel dsced Z's runnng triples, bg mtrs etc,

The Group S repped Z's are as quick, if not quicker.

That is satsfying.

 

That's the difference between a car that weighs close to a ton versus 670Kg! None of that happens, the pads and linings last a few seasons, and the drums have been on the car for many many years, it doesn't boil fluid, nor warp rotors. It does however leak fluid from the rear cylinders....