Tom's 40mm rear caliper & question RE prop valve

[PhoenixGTS]

I've had Tom's 40mm rear calipers on my 98 GTS for years. At first I had Kumho MX fronts with V700 rears so with regular compound fronts and sticky compound rears I needed as much rearward balance as possible so I removed the rubber seal in my proportioning valve. The fronts locked up befpre the rears. Then when I went to 18" Toyo R888s all around the rears were locking up just slightly before the fronts so I put the seal back in the prop valve. I have now gone to Gen III size Nitto NT05 tires (wear rating 200 so pretty sticky but not R compound but still the same all around) so the rears are a bit taller as they are 19". I have re-disabled my prop valve and went out for a test drive. Managed to get a lock-up (HUGE cloud of tire smoke in ther rear view mirror) but I could not tell what tires locked up first as the car stayed perfectly straight and stable. Went back around the block and could see by the width of the skid marks that the fronts locked first. So it looks to me like if you have Tom's calipers and the compund of tires all around but are running Gen III fitment 18" front 19" rears disbaling the prop valve is the way to go. Only thing I am worried about is that the rear calipers will adjust, i.e. the piston will screw itself out, and the rear brakes will start to work better moving the bias toward the rear.

 

[plumcrazy]

bump for tom to see.

 

[GTSnake]

I took mine out a few years ago and I need to put it back. I just hate having to bleed the system again when I do it. It's been so long I think I've lost the O-ring From what I learned after the fact is that by removing the O ring it lowers the braking force to the front brakes as a result the rears lock up first.

If you're trying to adjust the braking force I think you're better off getting one of those adjustable dial proportioning valve from BB Dave instead of playing with the O-ring. Just my $0.02.

 

[Tom F&L GoR]

Only thing I am worried about is that the rear calipers will adjust, i.e. the piston will screw itself out, and the rear brakes will start to work better moving the bias toward the rear.

The piston will screw itself out, but this is only the adjust the "starting position" for the parking brake. If it didn't do this, you wouldn't be able to pull the handle up far enough to engage the parking brake. The hydraulic system will still only push the piston with the same pressure as always. So the rear brake shouldn't be better or worse at any time or at any level of brake pad.

 

[Tom F&L GoR]

I took mine out a few years ago and I need to put it back. I just hate having to bleed the system again when I do it. It's been so long I think I've lost the O-ring From what I learned after the fact is that by removing the O ring it lowers the braking force to the front brakes as a result the rears lock up first.

If you're trying to adjust the braking force I think you're better off getting one of those adjustable dial proportioning valve from BB Dave instead of playing with the O-ring. Just my $0.02.

Actually, the "proportioning valve" is pressure limiting valve. It practice, it limits the pressure to the rear brakes while the front brakes always receive full pressure. Removing the seal allows the rear brakes to also receive full pressure. In action, the "prop valve" does not lower the braking force to the front, it increases the braking force to the rear.

The stickier the tire you use and the more weight transfer to the front the car gets, the more you want to keep the prop valve enabled. Less weight on the rear means less braking force is needed.

Note that we are discussing having "too much" rear brake with a single piston 40mm caliper! Food for thought. And indicative that the clamping force provided by a 40mm rear caliper is nearly ideal - may lean one way or the other depending only on minor car-to-car differences.

I would encourage people to bleed the rear brakes once in a while. Almost half of the cores I disassemble (about 160-170 pair now) have rust showing to some degree on the internal needle bearing and thrust washer just inside the brake lever arm. I don't have a great theory of how water enters the system, especially where there seems to be little fluid exchange, but the rust is often there.

 

[Steve 00RT/10]

I have run Tom's brakes for 50,000 miles with no prop valve in many different conditions including heavy rain, snow, and slush...plus several track events along the way. I have only ever run the standard Michelin tires. Zero issues. I will say that my last time on the track, I wore out the back pads(B3s) before the front. When I first took the valve out, I did a little testing to see which would lock first. On my car, the fronts still lock first with regular Michelin tires.


Steve

 

[PhoenixGTS]

BTW, putting that dang prop valve back together with that spring fighting you the whole time while bending over is close to the top of the list of the hardest things I have even DIY'd on my car. Doing that twice is two times too many.

 

[GTSnake]

BTW, putting that dang prop valve back together with that spring fighting you the whole time while bending over is close to the top of the list of the hardest things I have even DIY'd on my car. Doing that twice is two times too many.

Ha, that's exactly why I haven't put mine back yet....

 

[GTS Dean]

I studied the manual's cutaway drawing and decided just to remove the spring. The first time you apply the brakes, the spool will unseat and you will have balanced hydraulic pressure throughout the entire brake system. This was the 3rd mod I did to my car, after 5-point belts and K&N filters.

I don't think I've locked a tire (front or rear) since I did the 40mm rear caliper running 315f/335r rubber. On rare occasion, I'll lock a front tire if I'm running 275 width.

 

[PhoenixGTS]

I studied the manual's cutaway drawing and decided just to remove the spring.

CHEATER! I have to admit I thought about doing that after 45 minutes of fighting that spring, then bam - the threads caught and I was done.

 

[GTSnake]

What's the downside of removing the spring? Will it cause any problems?

 

[GTS Dean]

the spring ALONE is the only thing that controls any pressure differential in the system. Remove it and you're good to go.

 

[Roffle Waffle]

I studied the manual's cutaway drawing and decided just to remove the spring. The first time you apply the brakes, the spool will unseat and you will have balanced hydraulic pressure throughout the entire brake system. This was the 3rd mod I did to my car, after 5-point belts and K&N filters.

I don't think I've locked a tire (front or rear) since I did the 40mm rear caliper running 315f/335r rubber. On rare occasion, I'll lock a front tire if I'm running 275 width.

What size wheels are you running in the front to fit 315mm rubber?

 

[GTS Dean]

You can run them on 10s, but I have 12s that really work well. Lots of shoulder support and a good, square tread. Camber is more critical, though.