I haven't read the entire thread, so this may be redundant, but here goes;
Before I start; I remind you that most brake-fluids eat paint, so you know, if you spill some, wipe it off ASAP.
The booster works by introducing atmospheric air into one half of a pressure chamber, the other half of which has been evacuated. It can develop a tremendous amount of assist. My guess is around ten times whatever your leg is doing.
Attached to the brake pedal, is a pushrod that enters the control valve inside the booster, which is in charge of allowing atmosphere into the chamber. It works by sensing feedback in the M/C, thru the pushrod.
If, for whatever reason, the pushrod is poorly connecting the booster-diaphragm to the Power-piston inside the M/C, or the power piston is soft, then the control valve will not be properly activated, or it will be late to the party. Thus initially, there will be little to no assist. But, eventually, as hydraulic pressure builds, finally the control valve opens, and then you START getting boost-assist. But it takes extra pedal travel and a lotta leg power during the travel ...... because, essentially, it is standard brakes on a big-bore power-piston, until the assist arrives.
Ok so, now what I really want to talk about, is the reasons for this crappy situation.
So, I'm talking about a low to no assist situation.
Firstly;
I will assume that the physical parts in the system are 100% in good working order, and that then leads to the only question, which is why is the pushrod not back-feeding resistance?
There could be several possibilities, from the simple, which is
> a too short pushrod, to the more complicated
> still sponginess in the system.
Let's start with sponginess.
Sponginess could be either hydraulic or could be mechanical.
So then, in your situation; the first thing I have to see as the technician, is a high and hard pedal, cuz that's what the booster is expecting to see. About one inch of brake pedal travel is all I want to see before hydraulic pressure starts to build, and the pressure should build very rapidly.
So the thing is to figure out; is the problem hydraulic or is it mechanical, here's how I do it.
I jack the car up and remove all four wheels.
I remove the front calipers, and I take my trusty C-clamps and clamp the pistons into the bottom of the caliper bores.
Then I move to the back and completely disassemble the rear brakes, then clamp those piston-like slugs, into the wheel cylinders.
At this point, I have eliminated all the mechanical stuff, and hydraulic pressure should now build up immediately when I step on the pedal, which should be high and hard.
If it is not, then the hunt is on.
So then, the first thing I need to eliminate is any sponginess, which is air in the system.
As to the M/C;
inside the M/C are two power-pistons, one for the front brakes and another for the rear brakes. AND, those two are NOT mechanically linked. Instead, there is a hydraulic chamber between them, which when full of fluid, transmits the force applied to the first piston by the pushrod, to the second piston. The firewall-end piston operates the front brakes, leaving the one on the bottom end to operate the rears.
So then, you can see that if this chamber has air in it, then the firewall-end piston will slide right on by the port that goes to the front brakes, compressing that slug of air, until the rear brakes come on line. That situation then feeds back into the first chamber, thru the coupling chamber, and additional pedal travel will now activate the front brakes, allbeit with sponginess. If you have a spongy pedal, this is the first place to look, and there are only two ways to get that air out of there.
The easiest is to bench-bleed the M/C. see note-1
The second way is to pump fluid out until one caliper piston is near to popping out of the bore, then rapidly force the piston back to the bottom, and hope for the best. Usually, the air is faster than the brake fluid, and the result of this is a fail.
So now, retest for sponginess, which should be gone!
But if it is not, then you gotta go find the air; which with all the pistons clamped, should not be too hard.
Once there is no more sponginess in the pedal, you can drop the clamps and reassemble everything, then retest for a high and hard pedal.
If the pedal is again spongy; lets look at possible mechanical issues;
>In one case I had, the hoses were so tired, that the sponginess was due to hose expansion. I installed new hoses, and the problem was solved.
>Be advised; if the rear brakes are not adjusted properly, OR if you have installed new shoes on worn drums, you will lose the hard pedal due to brake force being used up in deforming the drums to the new shoes. See Note-2.
>Or if the back is OK, then, looking to the fronts;
>The pads must be free to follow the rotors; and to sit flat to them. See Note-3
> and finally, after the pedal is high and hard; it's off to the pushrod
synchronizing the pushrod length.
Now that the system is providing feedback to the control valve;
we can time the boost to it, so that we have progressive control.
Be advised, the window of opportunity is very small. The Pushrod has to park in a position, that allows brake fluid to return to the reservoirs, thru the compensating ports. See note-4
If the ports get blocked; as the friction surfaces wear, the pedal will drop. Eventually the pedal will be too low to be functional. It takes a long time so don't stress over it right now. Within the confines of that;
If the pushrod is too short, boost will be delayed; which means extra pedal travel and a tired leg.
If the pushrod is too long, boost may be instant, with the resulting head in the windshield Fifth-Avenue/Newport thing.
Between those two, and the C-port restriction, you can adjust the Pushrod anywhere to your liking. There is a sweetspot to be found.
I start with the pushrod as long as possible, without closing the C-ports. see note-4 , then begin to shorten it about 1>2 turn between tests.
read Note-5!
Ok so that's about it, lol, I know I said a lot, but I gotta tell ya, all this work will really pay off in the long run, as you release the stress of worrying about stopping. The car will be such a pleasure drive.
Happy HotRodding
Note-1
This is usually done, before the M/C is mounted. If you look inside the reservoirs, you will see holes. Usually a big one and a smaller one, but sometimes only a bigger one. Both of these are open into the Power chamber.
When you stroke the Pistons, on the bench, the power-piston slides by the holes, trapping fluid in the chambers to pump it out towards their appointed destinations. When you release the pistons, by spring pressure inside the M/C the pistons return to the designated parked position. This action allows the fluid to return to the reservoirs, thru those ports, reducing the pressure to zero. Now, on the bench, you can raise or lower the ends of the M/C to allow air trapped in the chambers to escape up into the reservoirs. Usually the air gets stuck on the piston sides of the ports, due to the surface tension of the liquid and so needs a lil help to escape. So I tickle the pushrod to help it. It may take many many tickles, depending on the size of the ports. To do this, usually the front of the M/C has to be lower than the rear, so that the air can travel to the high end where the ports are.
This makes it almost impossible to bench-bleed it on the car. And, you cannot be sure the air is all outta there, unless it has been witnessed in the bench-bleeding procedure.
> Now, if the M/C is already on the car and plumbed, Don't panic. Just unbolt it from the firewall and raise the firewall end, and tickle the pushrod, while observing the fluid in the reservoirs. Of course, since in my case above, the pistons are clamped, you cannot stroke the M/C, but you can tickle the air outta there.
Of course this is the part where I remind you that most brake-fluids eat paint, so you know, if you spill some, wipe it off ASAP.
Once the chambers are known to be air-free, you can bolt the M/C back on, and retest for sponginess.
Note-2
New shoes on old drums, do not fit. The curvatures are different and the shoes will only touch the drums in a small contact patch. The cure is to either buy new drums or to have your shoes radiused to fit the used drums.
But, when operating in this un-matched condition, hydraulic pressure will push the shoes out to hit the drums, and addition pedal travel will attempt to force the drums to conform to the shoes. In effect, the drums become very powerful springs...... which manifests as sponginess.
Note-3
If the pads are not dead flat, both sides, you will get sponginess. What I do is to take the two pads, face to face, and make sure they don't rock, then measure the two together with a caliper, on each end which should be the same no matter how I measure them. Then I rotate one pad 180* and remeasure.
Next I make sure the pads sit correctly in the brackets, and if the brackets have groves, I fix them.
Next, I prove whatever guidance system they have, which allows for pad-wear, is functional. If pins; they have to be straight, smooth, and easily slide in their rubberized sockets. And the machined "WAYS" have to also be smooth.
Finally, the leading and trailing edges of any pad have to simultaneously touch the rotor. I have seen bent brackets, and pads hanging up, that act like springs during braking, and produce the dreaded pedal-sponginess.
Note-4
as to the compensating ports being open;
There are two ways to tell;
Method #1
1) have a helper ready to
slowly and gently apply the brakes, but not enough to make the pedal hard,
2) meanwhile, you observe the liquid in the rearmost reservoir.
3) as the pedal goes down, you should be able to see a tiny bit of roiling down by the port.
4) as the pedal returns to it's parking spot, the roiling should return. Be advised that if there is air in the system being compressed, the returning fluid can cause anything from roiling to a 3ft gusher, depending on what your helper is doing! So again, go back to step-1, and remember, brake-fluid eats paint. Any amount of roiling is good
Method #2
Some C-ports are actually large enough that you can shine a light into it and see the shiny guts down there reflecting back at you. But you gotta know what to look for.
On the bench, before any brake fluid is in the reservoir, while stroking the power pistons, you can see the piston close the port as the shininess disappears.
With the piston parked, the interconnecting shaft is almost 3/8ths of an inch down past the casting. Whereas
When the piston goes by, it is right there.
Once the fluid in is in there, it is much more difficult to see.
Note-5
Be advised, that if you make the pushrod way too long, you will run the risk of breaking the control valve in a panic stop. Or if really long, it can be broken during installation of the M/C. When this happens, you will know it! you will get instant full assist just by breathing on the brake pedal, lol.
EDIT
BTW;
The fittings on the short lines that run to the Combination Valve are size-coded so you can't mix them up. The rearmost port on the M/C needs to end up on the front system and is not proportioned.
