Brake Questions - Help

With that engine and a manual trans, things can go bad in a hurry with an inexperienced driver. This is because with high-compression, the engine generates a lot of compression braking, and it all goes into the rear wheels....... making it easier for the hydraulic brakes to attempt to lock them up. However, it cannot do that without stalling the engine.Experience tells us WHEN to push the clutch pedal down to prevent those things from happening. And the P valve tries to make the car safe for all concerned, by limiting the rear line pressure to IIRC 85% of the front, and the factory sized the rear brakes to limit their capacity to about 15% of the total. As you can see, there is a tremendous amount of braking potential to be had in the rear brakes......... for performance driving.

Your valve is NOT a combination valve.
Your valve is strictly a safety switch/distribution block.
In a C-valve, that Texas shaped part will be an integral part of it.
As for the block at the back; I have never seen it be anything but a splitter.

215/60-14s, 340HO, and manual trans,
Should never appear in the same thread.
In late summer of 1970, I purchase a slightly used 70 Swinger340. I drove her from then until fall of 71 on those skinny E70-14s, which math out to about the same 215. That car was, bar-none, the most dangerous car I have ever driven. I couldn't wait to get bigger tires on it! I drove that 4-speed SG equipped 4-wheel drifting death-trap on those pizza cutters until fall, and then put bigger studded winter tires on it. The following spring I got me some wider wheels and the biggest tires that would fit in there; G60-15s on adapters. I drove it until fall of 1974, having gone thru 14 tires, mostly rears. I fell on hard times and let her go for $1300. She was all but burned out; being on the second engine the second trans and now with 4.10s in her. The body was almost mint, except for all the wipe out dings. There was a lot of those.... lol. G's were a joke too :( You know what Gs math out to?, about 245s. IMO;a stout 340 stick-car built for the street,with a DP carb, will not be safe, until 295s go under there and 245s in the front. If you have a vacuum secondary carb, you could probably get away with 275s. I tried those. Both on 60s and in 50s. They had enough rubber in the forward direction. But the failed the left turn blast-off from a light, putting me facing backwards every time.... with the 750DP. I tried a 600 and a 750 vacuum secondary carb. The 600 choked my engine so bad I thought it turned into a 318. With the 750 , it felt like a soggy 340. I HAD to put that 750DP back on.
But I digress.
I hope you have bigger tires on the to-do list...... sooner than later.

Once 275s are on there, she won't need any rear proportioning, with 7/8" wc's, perhaps not even with 15/16".. Mine didn't.