Spitfire & GT6 Forum

I've rebuilt the front brakes on my MK3 GT6 and now cannot get the master cylinder to pump fluid. Not surprising after sitting for a quarter century. The easy solution would be to buy a new M/C but I find that the later cars seem to have an NLA master cylinder. There was apparently a change in the 1973 production run. (One supplier has a NOS one for over $500).
Second option would be to rebuild the one I have. The kit is also very expensive.
The third option would be to use one from a slightly older car.
Anyone know why an older model cylinder one wouldn't work.

Tom

It has been a while but I seem to remember there was a difference in the M/C if a brake servo was fitted.

The M/C had a stepped cyl bore (two different diameters) which would be quite expensive to make.

Thanks, Tony. A stepped bore would account for the high price well as lack of availability. Perhaps poor logic but since I do not have a servo then I can use an earlier master cylinder?
Tom

I am going out on a limb here, but I think the change also involved rear wheel cyl size.

I hope someone will chime in that is more knowledgeable of GT6's than I am.

Thomas,
Check there might have been a change to metric threads on the latest master cylinder too. If so you would need to replace the lines to the PDWA if swapping in an earlier master cylinder.
Good luck,
Paul

---

TRF# 10423

Tom:

You have a 73GT3 Mk3 do you have a PDWA. The car should gave been equipped with one?
If you have one, is in proper working order?

Would help a lot to know the original GT6 Commission #, since Triumph doesn't go by model Years
Many vendors list the appropriate start/end Comm# for various parts.

Did you try bench bleeding the MC?

---

60 TR3A (red), driver
62 TR3B project
72 TR6, driver
69 Mk3 Spitfire 1296 EU setup
https://spitfiremk3.wordpress.com

Carter:Commission # KF22288 U 1973
Tony M: Yes, different wheel cylinder numbers in the parts catalog. Perhaps install earlier wheel cylinders to match earlier master? I]ll get the master cylinder apart soon and see if I can determine what's wrong. Maybe a kit will fix it.
Paul: I have not noticed any metric threads but will double check.
David: The car has a PDWA but the issue seems upstream of it. Condition of the PDWA is unknown.
Bruce, I have not removed the master cylinder but will do so today. I have cracked the lines at the MC and get no fluid.

Thanks all for the help.

Tom

Sitting 25 years eh? Yeah, I’d go out on a limb and say it could use a teardown, cleaning, and inspection at least.

I’ve found on my Spit, that from an empty condition it can take quite some time to flow fluid, even bench bleeding doesn’t always decrease time too much.

Tony, I’ll just ask you to take the high road today and refrain from your argument against Bench Bleeding, thanks.

I bench bleed with the MC installed on the car (yup, it can make a big mess if you’re not careful, and if you’re using anything but DOT 5 you’ll want to be extra careful).
By trying that you’ll at least confirm that the piston is actually pumping.

Good luck with it.

---

Steve
https://stevew10.wixsite.com/spit16

Well sitting 25 years, on top of whatever years can be added on that I would say the cyl needs dismantling with the hope it can be rebuilt. If the cyl is not pumping the seals are likely not contacting the cyl walls (likely shrunk or deformed from sitting)

Sometimes they can be tricked into sealing by slaming the brake pedal down. The shock pressure will force the seal into contact with the cyl wall. But I would not consider a reliable solution to the problem at all !

So new seals at least. But there is a very good chance the cyl wall have become pitted with corrosion. So while a seal kit might work, they might not last long at all.

OK. I now have it out on the bench. I can get the front circuit to pump, although weakly, and nothing from the rear circuit.
I haven't figured how how to get it apart yet, but will work on that today. (It seems quite corroded)
What I'd really like is to replace it with a new one. Perhaps someone could offer an opinion on using the earlier dual circuit master cylinder which is readily available and reasonably priced. Except for the rear cylinders, which seem to be different as Tony M pointed out above, I can't see any reason why the non-1973 master cylinder wouldn't work.
I know we are dealing with a rather critical component, so don't go out on a limb but do suggest a reason this change won't or might not work.
Tom

Toom, when you get the master sorted, there might be a issue getting the rear wheel cylinders. You have not said, but don't you have the non-rotoflex late model? If so, it's another whole kettle of fish.
The backing plate is different not only in size but in the holes in the plate, so just changing the wheel cylinder will not work as is.
If you do have the hardware for the self adjusting rear brakes, keep all of that and work on rebuilding that system.
BPNW does sale a master that is not over the moon in price and please just get a new one and if possible rebuild the PDWA. If you are going to stay original, you have a up hill battle but it can be done.
One thing you can do, not saying it's not going to cost about the same, is change the rear axles out for late Spitfire ones and use the 3/4 inch wheel cylinder back there. Others might have a better idea, but it is a option. I did it on the Spit6 and not a difference I can tell, but I did also take the PDWA valve out of the system.

---

Wayne
1970 TR6
2000 Jaguar XK8
1949 Triumph Roadster 2000
1978 Spit6 (getting all the good GT6 parts, all poly suspension and Spax shocks)

Thomas: you can replace what you have with a dual master cylinder. I replaced my original MC and PDWA during rebuilding with one using a new one from a Spitfire 1500

Replace the MC, and remove the PDWA. Pipe accordingly. The fire wall will probability need some modification to accumulate the extra length
very doable and a good upgrade.

Finally a bit of closure on all this. My late '73 master cylinder has been replaced or at least it is not the one listed for my commission number.
The difference, .05 inches, which turns out to be significant. ( I ruined a $45 repair kit trying to make it work.)
Here is a good breakdown that answered lots of my questions:
https://www.teglerizer.com/triumphstuff/brakecylspecs.htm
I tried to send the website owner a donation but the PayPal link didn't work.

Tom