IN THE VIDEO:
1. The guy is using a dual-diagonal master cylinder, which GMT400 do not use. "Our" master cylinders are split strictly front/rear. 2-front (both disc calipers) and 2-rear (both rear drums). Therefore, our master cylinders don't have a cast-in provision for the safety switch, either. "Our" safety switches are part of the combination valve, mounted to or at least near the ABS unit.
2. Like a million other Youtube videos, the guy
REALLY needs to work from a script instead of winging-it.
- the Q-T-U piston fluid has three exhaust paths:
- around the primary / secondary piston cups to the slave cylinder(s)
- through the check valve
- through the compensating port of the primary piston
Items (1) and (2) occur immediately or shortly after the onset of displacement of the piston train ("onset of motion of the brake pedal") and, as displacement progresses, the compensating port (3) will become exposed and shunt fluid to the reservoir.
This video is making me think. I hate it when that happens. I haven't dealt with a dual-diagonal brake system since Fido's great-granddoggy was a pup.
I have NO IDEA how the fluid from the big QTU cylinder gets into the secondary brake system. It should be blocked by the seal at the rear of the secondary piston--the lip seal that faces "backwards".
In the GMT400 master cylinder, that's not a problem. The secondary piston controls the rear brakes, and they don't need help from the QTU part of the cylinder. No QTU fluid flow to the secondary is no big deal.
SO: The more I think about this, the more convinced I am that there
IS NO fluid flow around that seal at the rear of the secondary piston. The pressure created by the QTU chamber pushes past the primary piston seal, just as described. This pressurizes the cylinder ahead of the primary piston seal, which then
applies force on the secondary piston causing
more movement than the motion of the brake pedal pushrod alone.
Put another way--the secondary piston moves faster/farther than the primary piston, because it's getting kicked in the butt by the fluid displaced by the QTU piston.
So there is additional fluid motion in the secondary due to the QTU fluid pressure/volume acting on the back of the secondary piston. It's not that important on systems split front/rear like GMT400s, because that additional pressure is resisted by the brake shoe return springs, but may provide some minor benefit. It's essential on a dual-diagonal system as otherwise one front caliper (the one connected to the secondary piston) would not have the benefit of the QTU chamber's high volume.
I'm pretty sure I learned something from this.
Try not to do that again, please. 
Upon brake application the maximum fluid pressure ("psi") against the Q-T-U piston is the lesser of
- the pressure required to force fluid around the primary / secondary piston cups, and
- the pressure required to overcome the check valve, and
- when the compensating port is exposed, the pressure required to force whatever volume of fluid is exhausted
Of all three above, the check valve determines the
maximum pressure ("psi") against the Q-T-U piston; the pressure may of course be less.
Yup.
Except for the part about fluid flow around the secondary cups.
Fluid flow when "quick-take-up" in the slave cylinders is taking place seems to be... (this is prior to exposing the Q-T-U piston to the compensating port):
- Fluid flows around the primary / secondary piston cups to the slave cylinders, e.g., to effect "quick-take-up" of the low-drag calipers' pistons, until
- Fluid flows through the check valve once pressure has risen due to (2a) the caliper pistons' resistance to motion (meeting the rotor) causes sufficient pressure rise or (2b) the MC's primary and secondary pistons' cups resistance to fluid flowing around them results in sufficient backpressure, which opens the check valve
Again, correct except for the part about fluid flow around the secondary seal.
Which brings us to what I suspect--but cannot prove--to be a common problem with the epidemic of "crappy brakes--low pedal" on 1500-series GMT400s. The check valve spring gets weak/broken, and so the QTU fluid blows uselessly into the reservoir rather than doing something productive, like moving the caliper pistons/pads up to the rotor.
Similarly, if the caliper pistons are semi-seized, they may require more pressure than the check-valve will permit, so again the fluid blows into the reservoir instead of moving the pistons/pads up to the rotor.
In either case, the pistons/pads don't move up to the rotor based on the low-pressure/high volume gush of fluid from the big master cylinder bore, they move based on the ordinary primary piston motion--resulting in a low pedal.
AND this is in combination with the screwy 254mm leading-trailing rear brake adjustment system of most 1500 trucks, which means the rear brakes are certain to be way out of adjustment--ALSO making the pedal low.
Did I mention that if the ABS solenoid valve that stops fluid flow into the accumulator leaks, it results in a low pedal?
If the "main" seals on the primary or secondary pistons in the master cylinder leak, it results in a low pedal.
There's a
bunch of different ways to get a low brake pedal. Some of them decrease brake effectiveness, some of them allow full braking power but with excessive pedal travel.
Functionally, only two ways to get an abnormally-high pedal--plugged brake plumbing (rubber hoses swollen shut, brake tubing pinched shut) or seized wheel cylinder pistons (generally caliper pistons, but could be rear wheel cylinder pistons.) A high pedal almost always results in poor braking power.
I'm trying to explain to myself why the Q-T-U MC offers, in my experience, resistance to pedal motion before the brakes become effective (before the pistons are pressing against rotors). This is not very noticeable under light application but is extremely noticeable during panic application, from what I've experienced.
Some of that is probably in the booster, some of that from the master cylinder being unable to relieve QTU pressure through the check-valve fast enough. Maybe some of that is not being able to push fluid through the ABS and combination valve fast enough.
I can't say that I've noticed that effect on my QTU '88 K1500--at least
not more than I've felt it on every other vehicle I own.
www.jalopyjournal.com
