26 replies to this topic

[lx308]

Hi all.

I've had my master cylinder rebuilt using 1 of these kits, http://www.earlyhold...-torana-square/

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Needed the correct primary piston from the kit as the 1 I had wasn't correct.

 

Now, the original primary piston has holes in it behind the cup & washer, the piston in the kit doesn't. We're thinking this is vital, but not sure, as we're having trouble achieving an acceptable brake pedal after bleeding the brakes. Photo below shows the holes depicted in the piston (number 3) from the parts book.

We've bench bled the master.

Cheers

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[dattoman]

If they have provided a soft sided compensating seal then the holes won't be required

Most pistons don't have the holes.... you could drill them if you want and try again

Did you check/set the piston length of the bare alloy one ?... if you've had the cyl sleeved and they have drilled the holes in a slightly different spot this can be an issue

[lx308]

Thanks Datto.

Haven't checked/set the piston length. Surely the brake shop would do that as part of their reassembly?

I spoke to the supplier & he told me the new piston is the later design, so I'm assuming it's got the correct seal on it as it comes assembled. 

We also spoke about how far the pushrod protrudes out of the booster, he reckons 30mm, so I need to check that. He also told me to check that the rubber disc is still attached to the rod inside the booster.

I checked the depth of the rod hole in the piston, which is correct as this why I used this kit,  to get the correct deep piston.

So I have a couple if leads I need to look at.

[dattoman]

Check that piston depth

The info I have is rod sticks out past the cannister by about 25mm 

It shouldn't preload

Should have about .5mm clearance

[lx308]

Check that piston depth

The info I have is rod sticks out past the cannister by about 25mm 

It shouldn't preload

Should have about .5mm clearance

Got any directions for checking/setting the pistion depth? I not know of such wizardry.

[dattoman]

The proper way is with a H gauge... which you could make yourself

 

 

[liswar2]

On a seperate note there were 2 pushrods 1 for lc and 1 for lj, depending on the master cylinder , the difference was the length ,
I found this out the hard way years back, the resulting problem was only apparent after I had driven the Torana for a while then the vacuumed pressure in the booster would cause the brakes to lock on, after being turned off for a hour or so they would free up, fortunately I had a couple of double diaphragm brake boosters in the shed and with a little R&R discovered the problem, my big nut was rebuilt by PBP, ,
Hope your up and running soon Tony,

[lx308]

Cheers Datto, clear as & simples. Might see if the brake shop has 1 they'll lend me. At least now I know what I'll be talking about

 

Hi Wazza, that's the reason I bought that kit in the link, to get the deep piston.

Will get this thing on the road soon!

[Ando]

I just did this the other day after getting booster & MC back, reconditioned.

 

I had to lengthen the push-rod 23mm to suit another MC fitment. 

 

Use the depth gauge on your verniers for the MC & a steel rule across the booster hole.

 

Then add & subtract accordingly, you can work it out. 

 

My push-rod has a 5/16 unf thread in it so I found a bolt, cut it to length & ground the head down.

 

I asked my brake re-builder to do this but he obviously forgot! 

 

His solution, was for me to fix it. To cut down a bolt & place it in the gap between the rod & piston. Go figure!!

 

I can't believe he would say such a thing & I didn't take his advise.  

[lx308]

Yea, I asked the local brake shop if I could borrow his H gauge, he laughed & said hd sold it 20 years ago with a lot of other gear he never used. 

Calipers will do me fine I reckon, might get enthused to give it a crack today.

[jd lj]

I had a thread going a few years ago about the different push rods lengths for LC and LJ with both the std booster and XU-1 booster sizes listed.

When I had my big nut master cylinder rebuilt years ago and went to fit it it turned out that the after market master cylinder I was previously using was the LC type so I had to get a threaded extension for the push rod, which just meant that I unscrewed the old tip and screwed in the new one. I had to play around with the length to get it right though.

[rodomo]

I had a thread going a few years ago

http://www.gmh-toran...ush-rod-length/

[lx308]

So here's my working out.
43mm overall piston hole depth to the flange face that inserts into the booster. Minus the the flange face thickness of 15mm, as it's inside the booster. Leaves an effective piston hole of 28mm.
Pushrod sticks out 25mm. Is 3mm too much play?

Also, I'm assuming that's the reaction disc I can see at the end of the tunnel there?

[Ando]

Tony, I see 15mm on your MC flange thickness but what's does it measure from MC mounting surface to the end of the flange? 

[lx308]

The flange that goes into the booster.

This peice of the master that the calipers are around.

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That's 15mm thick

[Ando]

On your sketch. You have 15mm. There seems to be a missing dimension between the 15mm flange back to the MC main body bolted surface.

 

Is it about 4-5mm which should be added onto the 15mm before subtracting from piston depth?

[S pack]

On your sketch. You have 15mm. There seems to be a missing dimension between the 15mm flange back to the MC main body bolted surface.

 

Is it about 4-5mm which should be added onto the 15mm before subtracting from piston depth?

Just checked an alloy PBR m/cyl and Tony's measurement of 15mm from mounting flange to end of cyl bore housing is correct.

 

Although I got a total installed primary piston pushrod bore depth of 41mm - 15mm = 26mm effective depth for booster pushrod with m/cyl bolted up to booster.

Edited by S pack, 19 September 2018 - 08:49 PM.

[lx308]

Just checked an alloy PBR m/cyl and Tony's measurement of 15mm from mounting flange to end of cyl bore housing is correct.

 

Although I got a total installed primary piston pushrod bore depth of 41mm - 15mm = 26mm effective depth for booster pushrod with m/cyl bolted up to booster.

That's the better description, mounting flange to end of cyl bore is 15mm

[S pack]

Yes that looks like the reaction disc in there Tony.

That reaction disc doesn't look real healthy. Perhaps it's just a bit too much rubber grease?

Also looks suspiciously like a drum brake Master Vac reaction disc.

See if you can get it out without losing it inside the booster, alternatively remove the Master Vac from the vehicle and hang it upside down to help it to fall out through the hole.

[lx308]

Might pull it off & do that, cause sure as shit, if I try to just remove it it'll end up in the bottom of the booster anyway

[S pack]

Might pull it off & do that, cause sure as shit, if I try to just remove it it'll end up in the bottom of the booster anyway

lol, I think most of us have had remove a master vac at one time or another to fish a lost reaction disc out of the vacuum chamber.

 

When you get a chance post up some good pics of the reaction disc (both faces) and a pic of the exposed end inside the valve body. (The bit the reaction disc sits against)

Edited by S pack, 19 September 2018 - 09:29 PM.

[lx308]

Might even be a spare disc in there already

[Ando]

Ok the sketch confused me which isn't hard for that to happen lol

[lx308]

Ok the sketch confused me which isn't hard for that to happen lol

Don't worry, my sketches can confuse me too

[lx308]

If they have provided a soft sided compensating seal then the holes won't be required
Most pistons don't have the holes.... you could drill them if you want and try again
Did you check/set the piston length of the bare alloy one ?... if you've had the cyl sleeved and they have drilled the holes in a slightly different spot this can be an issue

So Datto, if a soft sided compensating seal hasn't been used would any fluid come past the seal at all? I've been trying to suck fluid using a vacuum pump at the rear & I've never pumped for so long without a result...
I've set the pushrod at .5mm