Sprint ST master cylinder- how to tell if you have the right one

[TheRedOnesAreFaster]

Having the mushy brake problem?  Wondering if you have the right master cylinder?  The proper master cylinder on the ST (at least the ABS versions) has a 5/8" inside diameter.  Many had a MC with diameter of 14mm (I have no idea why they switch from metric to English) incorrectly installed at the factory.  Here's how to tell if you have the right one:
 
Look between the MC and the handlebar, right underneath where the hose from the reservoir joins the MC.  You may have to move around a bit to get a good viewing angle.  There is a circle with a number in it.  If the number is 14, you have a 14mm MC.  If it is 5/8, you have the 5/8" MC.
 
If you have the ABS version (again, I do not know if this applies to non-ABS versions), with the 14mm MC installed, the dealer should replace that one with a 5/8" MC under warranty.

[Capirex]

I've never had any problems but I just looked and I have the 5/8".  Has it been determined that this was the problem?  I'm not real good with english metric conversions but there isn't much difference between 5/8" and 14mm is there?  Isn't an inch like 25mm?

[TheRedOnesAreFaster]

I've never had any problems but I just looked and I have the 5/8".  Has it been determined that this was the problem? I'm not real good with english metric conversions but there isn't much difference between 5/8" and 14mm is there? Isn't an inch like 25mm?

No, there's very little difference in the diameter.  ⁵/₈ is 15.88 mm.  But that is an increase in diameter of about 13.43%.  The volume of the cylinder is determined by the formula:

.25 x pi x d² x height.  

Height is the length of the stroke of the piston (I don't know what the actual stroke is.  For purposes of this discussion, lets assume it is 1mm.), pi = 3.141, d = diameter of the piston.

So, for the 14mm piston:

.25 x 3.141 x 14² x 1 = 153.909mm³ (or ml).

For the ⁵/₈ piston (15.88mm):

.25 x 3.141 x 15.88² x 1 = 198.019mm³ (or ml).

This is an increase of 44.11mm³, which, as a percentage, is a 28% increase over the volume of the stock piston.

So, a small difference in diameter of the piston leads to a big increase in the flow volume (if we assume the same stroke, and we have no reason to think the stroke is any different), hence you don't need to move the lever nearly as far to move the pistons.  With the 14mm, you simply weren't moving enough brake fluid to get maximum pressure on the pads, thus the "spongy" feel.

I had the mushy brake problem with the 14mm MC, the larger MC solved the problem.

Ar any rate, Clint (the Life Cycle service manager) told me that the smaller pistons were improperly installed on a number of Sprint STs at the factory, and the replacement part and labor were covered under warranty.  I don't know if this applies to non-ABS versions, but I will try to find out.

[Capirex]

Wow.  I just had a highschool physics flashback.  

 

[Longpath]

"Should"  Awfully open, isn't it?


Are there any teeth to this issue or is it just a "should" issue still?  I have a non-ABS 14mm and the brakes suck the big one.  Like most dealers (I assume) mine will please ignorance until something official is produced.  

If you have something official, please share it.

[TheRedOnesAreFaster]

"Should"  Awfully open, isn't it?


Are there any teeth to this issue or is it just a "should" issue still?  I have a non-ABS 14mm and the brakes suck the big one.  Like most dealers (I assume) mine will please ignorance until something official is produced.  

If you have something official, please share it.

I don't think there is an official bulletin on this problem.  If there is, then I'm unaware of it.  If your service guy won't rectify the problem, contact Triumph directly, or try another dealer.  The dealer should want to fix it, because they do get paid for it.

My service guy had no problem doing it, and Triumph paid for the part and the labor.  

[Longpath]

Good for you and your dealer.  Based on previous interactions with both my local dealer and Triumph USA, I will seek alternative remedies unless there is truly a bulletin or recall.

[TheRedOnesAreFaster]

Good for you and your dealer.  Based on previous interactions with both my local dealer and Triumph USA, I will seek alternative remedies unless there is truly a bulletin or recall.

I've never dealt with Triumph USA, but I have been told by a number of people who have dealt with them, that the secret to getting results from them is to be polite and calm.  If you call and act like they owe you something, or give them a bad attitude, they will be less than helpful.  Be respectful, and they will help you out.

I've been told this by a number of people whose opinions I respect, and who have had dealings with TUSA on a regular basis.

It never hurts to give it another shot.

[Longpath]

Sigh....


I was, they were just unwilling to help.  Sometimes you just learn your lessons.  I'd buy a Triumph again, but will continue to do my own work.

[TheRedOnesAreFaster]

Sigh....


I was, they were just unwilling to help.  Sometimes you just learn your lessons.  I'd buy a Triumph again, but will continue to do my own work.

Well, at least you have a non-ABS version, so you can bleed your own brakes.  

How far from Kalamazoo are you?  Is it worth a trip over here to get it done?