47-55.1 Pressure bleeding

[joedoh]

I am going to say, Tuff Stuff likely does not make their own master cylinders. maybe they make their own prop valves, but masters are usually OTS (off the shelf) items because of the liability involved (and production needs! casting machining etc, thats a lot.). every GM master I have used the reservoir closest to the booster is the front output.
swap the lines. like ds says, nothing hard about switching the lines. might be magic.

[dsraven]

by the pics, what you have there is what they call a combination valve. it is a valve block with 3 different valves built into a single block. the "front" part of the valve, where the little black knobbie is and where there are spots for 2 brake lines to connect, is the metering valve which slows down the fluid going to the front discs until the rear drums can apply and build a slight pressure. ensure those 2 ports are connected to the front brakes. the rear of the valve is the proportioning valve which meters/limits the fluid going to the rear brakes under hard brake applications so the rear wheels don't lock up. the centre section is a brake failure section, or pressure differential valve section, which will close off one circuit if there is excess flow going to that circuit, like a hose or a line failed. this also has an electrical switch that turns on the brake failure light on the dash and is a reason why, when bleeding the system, sometimes the light comes on and it is suddenly hard or impossible to bleed one circuit of the brakes. the rear section should have a single port which needs to be connected to the rear circuit.
here is a link to a video explaining the operation
https://www.youtube.com/watch?v=3z1q7MVBs08

another with a cutaway and explanation of the combi valve
https://www.youtube.com/watch?v=3z1q7MVBs08

and an instruction from CPP on how to connect their combination valve to the master cyl. they seem to run their system with the rear connected to the primary on the master (closest to the booster).
seems like a quick, fairly easy job to try swapping the front to rear ports on the master to combi valve and see if your system works properly.

https://www.classicperform.com/Instructions/PDF/PVK.pdf

one with a good pictorial of the typical system found on hotrods.
this system has the primary connected to the rear port.
http://www.how-to-build-hotrods.com/brake-system.html

and another from POL which shows the rears connected to the primary. it seems like if you look on google there are systems set up both ways and systems are connected differently depending on who you purchased from. long story short, if it was me, and it don't work like it is set up currently, then I would swap the master outlets and try it the other way.
https://www.performanceonline.com/bl...-system-setup/

one other thing I was gonna suggest is to add a vacuum accumulator in the line with a check valve set up to hold vacuum on the booster side. since your booster is likely a smaller diameter and doesn't have the vacuum capacity of a larger booster. some vehicles have these under hood already so you may be able to source something from a wrecker if you take a coffee and some time at the pick n pull yard. since the vac line is fairly long from the engine down to the booster you could have a slow response or lose quite a bit of vacuum reserve in that long line under full throttle or engine stall situations because the check valve is waaay down at the booster so the vac line drains when the engine vac goes down. an accumulator with a check valve close to the engine would hold a reserve vacuum in that long vac line. sorta like the old vacuum wiper scenario, its all good till the hill.....

hope that helps. first ensure the combi valve is plumbed correctly, then check the master outlets. swap as you see fit, they are short lines to make/buy or you could have some flex lines made for easy swapping so you don't take time to fab a new line and leave the port open to make a mess and possibly allow air to get in the system.
you know you gotta get back to us with the results, right..........?

[dsraven]

sorry, forgot to put these links in that show the primary connected to the front brakes.
https://www.speedwaymotors.com/the-t...reet-rod/28869

https://www.motortrend.com/how-to/br...ot-rod-basics/

[72Mountaineer]

Thanks for the opinions and info. I spoke again to Tuff Stuff tech today and he said that the MC/Combo valve is plumbed correctly as their recommendations. Another tidbit was they recommend 18in. of vacuum to the booster I am using. In the spring I took a vacuum reading at the base of my carb (I replaced a power valve) and got 14.5". How much can an accumulator and check valve help? Is the pic I posted a check valve?

[dsraven]

That looks more like a filter
Inline check valve doesnt add to the existing vacuum, it just holds that vacuum in the line if you put the check valve close to the carb. Otherwise that whole long vac line will lose vacuum when you step on the gas. Its like a compressor with a bigger tank than it used to have.
I get it, they say connect like this, but if you swap the lines and it works better then no harm no foul. Guys connect then no harm no foul.

[dsraven]

sorry, at the end there I meant guys connect them either way.
here is a pic of an inline check valve. you would plumb this in near the carb to keep the line from there down to the booster charged with vacuum even if the engine is shut off.
https://frsport.com/products/ate-990...iABEgJlm_D_BwE

if you feel you need more vacuum for better boost you can install an electric vac pump
https://help.summitracing.com/app/an...or-canister%3F

you could easily check that filter if you want, to see if it is a check valve or not. simply put, you can blow through one direction only. they started putting the filters in back in the 70's to stop crud from getting into the booster and deteriorating the diaphragm. I don't know if they have a check valve or not.

this is what a vacuum cannister looks like
https://www.amazon.ca/Competition-Ca...20244615&psc=1

[dsraven]

smaller booster may benefit from more vacuum but that shouldn't mean the brakes don't work properly. a brake set up should function properly even if the vacuum is lost. without vacuum the pedal should feel firmer or harder to push but the brakes should still function.

[72Mountaineer]

First day back in the garage in a few. Went to upullit yard Sunday, got a check valve for the vac line, nice easy to get on older Nissans, after install I'm getting 18" at idle at the booster, just what the tech said was needed.. Did some rear brake adjusting, no more pedal but brakes seem a bit firmer, just a little bit.

[dsraven]

do you have a park brake?

[dsraven]

if you have a park brake, set that, then try the brake pedal and see if it is higher.

[dsraven]

https://www.motortrend.com/how-to/hy...tem-conversion