The 3980797 pressure switch/ proportioning valve assembly most definitely has regulator valve in it . You cant see the setup inside very well from the outside. as the later models. from the outside it looks very uncomplicated, but that is a illusion. But if you disassemble it there is a a set of springs, a valve shaft some o seals and a switch that has a plunger switch in it that ride on the valve shaft. The unit is really a proportioning valve. The master cylinder does not regulate front vs rear brake line pressure. The two lines from the master cylinder are balanced in the block. Each line master goes to one end of the block. The internal sliding valve shaft has springs at each end that are calibrated by screws, and they are the same as each other force ratios without the screws. The valve shaft free moves along the internal cavity. as brake pressure is applied. The valve shaft has groves cut into it that align with the output fluid line holes of the block. as the valve slides one way it lets in more pressure for front, and less pressure to back line . As it slides the other way it will allow less pressure for front and more for back. The proper pressure front and back is set by the central position it settles to. The springs are specifically calibrated to position /proportion the pressure to front and back to the proper amount at center. The end caps holding springs are adjustable to calibrate in finer detail. They allow the spring preloads to be set differently which acts like having different spring with different rate ratios. or simply restated as having different springs. The reason they are adjustable, and not fix rate ratio springs which would be easier, is because springs change rate ratios over time and age. having adjustable allows them to be reset to the proper rate by adjusting preload on them at the screws. to bring them back to the same balance as they will age differently. Once calibrated. the valve slides to the desired position as brake is applied. Remember the pressure coming out of the master is not regulated , as a result the front to back pressure will be not the same on each line. slightly different for every master made, as well, manufacturing variance. The valve set them all to be the same.. . The light switch has a plunger pin that rides on the valve shaft, in the center of it. the shaft tappers off side to side with a peak in the middle. the switch plunger rides on the peak compressing the pin up and off the contacts, for the everything is ok position. . if the valve slides to far one or the other way the pin is extended because the shaft creates more clearance for it too. Extend enough. and it will make electrical contact inside itself illumination the brake proportion valve warning light. If your light never goes on, it may not be a good sign, and most like is not. the odds are against you due to age. . The valve could be stuck by corrosion or the plunger pin on light switch frozen corrode stuck in the compressed up state state, or light switch broken, or wire broken. most cars with original equip this old they dont work... If it does work everything is really ok. But on these old cars ok is the least likely. There are tons of these cars driving with poor rear brake or front brake pressure and the light says ok. That is a illusion most of the time. many people spend tons of money changing master cylinder, or calipers ,pads, boosters, or lines and fine they get only slight or moderate improvement. because the light didnt go on and they thought it was everything else but the valve... .a light that never goes on, is not a good sign. They are not just a total failure warning light. They should flash briefly every once in a while as the brake is applied hard and fast, as the valve moves to proper position when in use. particularly in quick heavy brake pedal apply... Showing good operation of the proportioning valve to slide to position, and showing switch to detect slight moments of pressure not centered on lines. Hope this clears things up. If it does not do this, you should adjust the springs on one or other side to see if the light ever goes on. If it does, then set it back, to where it was, and then back off just so the light will flash a moment on heavy fast brake apply.

 

These guys all think (see link below) that prior to 1974 there is no control on a C3 for the front to rear brake bias...I don't believe that! I did look at diagrams for the earlier C3's distribution brake fluid box and it is exactly what you describe above^

https://www.corvetteforum.com/forums...problem-2.html

The smaller rear caliper piston bores are not enough to control rear wheel lockup under very hard braking and doing some research I found experts that wrote that there is no way that a C3 could have no rear brake bias control to prevent rear lockup in the rears in a panic stop with a rear 4 piston caliper and a 12 inch rear vented disc with a rear pad the size of the front brake pad.

Appreciate your write up!

 

Im sorry to see they all are so very wrong . What I wrote is very correct, thank you. I'm sorry because as a result of arrogance or denial of good help, there are people driving around with poor breaks they think are good,or great, and they have no idea just how good these breaks really are when operating correctly. it is night and day. I am a corvette expert listed in the front credits of the corvette black book starting in 2016 editions.. I have been doing this more than 40 years and I am a former NASA scientist, technical director for over 20 years. I have advanced degrees in engineering as well, that helps because it is all been applied to my auto skills throughout my life as i have always been a car restorer as well. . The pressure is not important as they think when you look at the size of unit. It is a illusion to think it cant handle high pressure. .The valve can cut pressure or increase it by the valve controlling the how it changes the size of the opening to the output line, in a piston like fashion.. It basically works similar to a water valve piston. These valve parts are small,and the block is thick for the high pressure ability. As the ends of the valve plates (which are machined like a dual piston on a single shaft ) cover over the holes of the output opening they obstruct flow. The flow is metered by ever so small movement of the piston to cover the opening. . This unit can handle well over 1000 lbs of hydraulic pressure. . Ill attach a picture of the real unit we are talking about here, and what it looks like apart. To prove it has many parts in it. It is not very apparent they are in there as the retaining sleeves and screws and hidden by being deeply recessed into the device. You have to be a bit of a fine machinist to take apart, similar to the complexity o transmission valve body detail. The valve are very much like the valve body pistons inside a transmission valve body and handle similar high pressure , even though very small, simply because the use a piston like cutoff flow design or whats called piston metered orifice . Thank you for your support and wisdom.

 

there is way too much engineering into a brake system, no brainer to buy new parts made for your year, replace your brake lines front to back, replace your hose ends

 

That is unfortunately not the correct answer where brakes are concerned. The proportion valves are probably one of the most critical parts in your brake system to assure the brakes have full power at each caliper piston. You did not even list it in your statement, which shows just how overlooked it is. As I stated it can and does malfunction in many ways where even when it is malfunctioning you believe it is not. I will bet more than 70% of the cars out there in the road with original valves , the valve is not working and is stuck in a fixed state. The proportion valve You can buy today are not exact as original. The internal parts are smaller, and modified how they work. I have tried and tested several new ones, and have yet to find one that works properly. . They are made by inferior offshore companies and more often than not do not even work correctly right out of the box. Even when they do work, they have a much smaller internal cavity. The aftermarket part internal parts do not even fit in the OEM part. . under boyle's laws, P1V1=P2V2. So you can see the output pressure p2 is a function the volume v1. To understand how to apply this to this issue, you consider only the change from using OEM valve vs the smaller internal cavity replacement valves you can buy today. Consider them as the p1 v1 pressure density source. If you decrease the internal volume of proportioning valve, V1 from the original size to something smaller than OEM, , then you are going to decrease the pressure at the output brake line volume V2 which remains unchanged. You can still buy factory spec internal parts for the OEM valves to rebuild an original unit. They are after market parts, but they are made to work in OEM blocks. A much better way to go. I have written more than enough that anyone should realize, this little part can and does make most C3 not have proper brakes and nearly everyone does not even know it. Also it was mentioned the valve maybe can't cause rear break lockup. But I assure it can, the most common failure is a weak line front, back, or both. Anyway to get back to your response. replacing all parts does not always work where there is 1) non equivalent parts, or 2) something called a critical component with whats called "Blind failure mode". A blind failure is something you cant detect in the failure detection system. In this case there are several failures the red light wont show. Getting back to changing parts. I recall once someone who had a brake problem and had been to 3 repair experts. After $1500 and nearly ever part changed on the entire car changed, it still did not function well, or proper pressure. the pedal was soft at times. I was asked if I could please fix it for him once and for all. I recall getting into the car, starting it up, and stepping on the brake. I instantly heard the problem. Yes heard the problem. I could hear the air leak ever so slight from the back of the brake booster. It was intermittent too. Not on every step. Bad booster diaphragm. The funny thing was, it was a new booster. The so called experts had changed every part. A madman process of removing the bad part they never could or did isolate., but in the process of fixing it, they added a new part which caused a similar failure for a completely different reason. This happens very much on old cars with all these reproduction parts. It is always best to find the real problem, and fix only it. Good luck and thank you for your inputs. Also keep in mind, the master cylinders are made with dual pistons and the primary and secondary are not the same feed cavity volume. primary is threaded 9/16 20 and the secondary is 1/2 x 20 . Another clear reason for the proportion valve. It balances them to be equal. Also consider the fact the master cylinders are all made to SAE J1153 specifications, which is they must deliver and be tested to 1500 ps1 +/- 100 psi.... Your brake booster gives you a typical 6:1 ratio. Stepping on the brake with 200 lbs foot force is 1200 psi.

 

what I really meant was:
after 40 years, the o rings have lost their elasticity, the springs have lost their tension, the pistons and bores are worn, the steel lines are rusted, and the hoses are ready to fail

If the replacement valves are as you say inferior, how would I know. It is the only vette I drive

I actually thought the valve body was steel, and the lines would not come off. I am sure I thought I was going to break off the lines going into the valve when I changed the old brake lines