Back up the bus. Waaay back.
Firstly; the P-valve limits rear pressure in proportion to the front. So when/if you install one,you will have even less braking power. The front brake hydraulics are routed straight through it, no detours. If you have front brakes at all, then the front half is working for sure.
Next is this; The 73 system is adequate for at least one very hard stop from 60mph; with performance pads, several stops' and it doesn't matter which M/C is on it or if it's boosted. The booster just makes it easier for persons with less powerful legs. And the same goes for M/C bore sizes.
So the first thing you need to do is figure out what exactly is the problem. Is it hydraulic? Or is it mechanical? or is it Materials?
The place to begin testing is the pedal. With the engine off, step on the pedal. Do you have a hard pedal?Is it hard very near to the top of it's parking spot? or down at the floor? or someplace in the middle. Do you have to pump it to make it hard.It should get hard within the first 1/3 of it's total travel.
If it doesn't get hard, there are several possibilities for that. A) is air in the lines. B) is faulty pads or pad installation C) is flexlines or other.
If it does get hard but it's way down there; that would probably be a bad rear brake adjustment.
If it takes several pumps, the rears for sure need adjusting.
So let's say you have a hard pedal on the first and only pump, and in the top 1/3 of the pedal travel. What does this prove? A)There is no air in the hydraulic system, and B)the rear brakes are adjusted properly, and C)the pads are correctly installed, and D) the pads are not bent or worn crooked, and E) the sliders are probably not stuck, and F)the flexlines are ok, and the wheel bearings are close to adjusted right
See how much this test can prove? Try it.
OK so let's get back to it. Again, let's say the pedal test is 100%.
Next is mechanical. The rotor surfaces need to be reasonably smooth and parallel to eachother.The pads need to be reasonably close to the same width from front to rear.They must be reasonably smooth and absolutely flat. They must be clean and uncontaminated. If they have ever been soaked in brake fluid, throw them away.It is nearly impossible to make them serviceable. Same goes for the rear shoes' except of course the parallelism, and additionally , if they have ever had gear-oil on them they are useless. Rear shoes, to be effective must be closely matched to the drums. That means when you drop them into the drum, they need to contact the drum over most of their LENGTH. And the drum does not work very well if it is bell-shaped, that is to say the inner diameters are different from inside edge to outside edge.
And finally is materials.Available are several different kinds of materials from the common stuff to ceramics. Some materials are more effective at high temps. Some require more leg effort. And some require different amounts of leg power under different operating temps. And lastly; Ignoring contamination,some manufacturers materials are junk.
Now let's talk about M/Cs
There are basically two kinds of dual-chamber M/Cs and several different sizes. All can be used on your A-body. The biggest difference is that M/Cs designed for all-drums have residual valves installed into the outlets right in the ports. These valves maintain a small amount of hydraulic pressure in the system to keep the wheel cylinder cups in contact with the bores, to prevent the wheel cylinders from leaking. These hark back to the days when wheel cylinders did not have energizer springs inside them to do the same job. Now these R-valves must not be in the front system of a disc brake equipped car. They will keep the pads jammed up tight on the discs. Not so tight that the car will not move, but tight enough to fry the pads.So that get's us to the disc/drum M/C. This is the biggest difference between the two; the all-drum will have 2 R-valves while the Disc/drum will have just one.
Next let's talk bore sizes. The bigger the bore, the more leg power will be required to stop the car. And the pedal travel req'd will be less than a smaller bore. The smaller the bore, the more pedal travel will be req'd but the effort is less. That's it;no magic here.
And finally is the booster.
The booster is an assist device. It assists your leg by amplifying your leg power. It uses atmospheric pressure against engine vacuum to accomplish this. No or low vacuum means no or reduced assist. The booster is slightly adjustable in the timing of the boost, by adjusting the pushrod that connects the footpedal to the M/C. But wait; did you get the part about no vacuum equals no boost?
Heres how to prove the booster is working. Or not. Get in the chair, and stomp the pedal about 5 times, letting up all the way after each application.This is to deplete fully any stored vacuum. Ok, now step on it with about 30 pounds of force(not critical), and start the engine. If everything is working right, the pedal will fall between 1 and 2 inches,within 2 seconds or less. This proves the booster is working and that the engine is making enough vacuum at idle. If it does not fall, rev it up to about 1500rpm. If it now falls then there is not enough vacuum at idle to make it work, but there is at that 1500 rpm. Now take your foot off the pedal and let the engine return to idle. Step on the pedal again. Does it have the same or similar travel,as when it was at 1500? If yes that means the checkvalve is working, and that a vacuum is being stored.Now engine still idling, step on it fairly hard maybe 5 times. Do you feel the pedal-travel changing with about the same amount of leg-power? That means again the engine is not making enough vacuum to keep up.It does not mean the booster is bad.It just means you can expect just one assisted stop when the engine is idling. This is worse on automatic cars, but with stick cars, that have a higher idle, and are often downshifted as the speed drops, it's almost unnoticeable. Lastly here, is boost timing. If the pushrod is too long, boost will be too sensitive, and under certain circumstances it's possible to break the control-valve inside the booster, and it's possible to defeat the compensating port. If it's too short, you will have to step really hard on the pedal to reach the piston in the M/C. This can also break the control valve. And it will make the car SEEM REALLY HARD TO STOP. Sound familiar at all?
We'll talk about the P-valve if the need arises.
Conclusion; BEFORE you buy any parts, work your way through the diagnostics.In the end it may just be the pushrod adjustment. But if I hadda told you that in the beginning, and something else was not right, and you broke the control valve, you woulda wanted me to buy you a new booster; am I right?
Ok now Big, have fun......
rayer:
