brake failure due to proportioning valve malfuction

I nearly crashed my 1986 Crown Vic because fluid blasted out from under the rubber cap on the rear end of what I think is called a proportioning valve. It is located about a foot below the master cylinder and has 5 brake lines attached to it. I was told they last forever which apparently isn\\'t true and that the only place to buy a new one is a Ford dealer for about $200. Is that my only option and is this a common problem? About a month ago it blew a rotted steel brake line so I replaced the entire system except this valve based on what I was told about the valve plus I never heard of one failing. I checked the fluid level often during that period and it wasn\\'t leaking until a hard brake stop at the scene of my wife trapped in my 88 Ranger on its roof due to a jerk not securing a load on a pickup (she is ok) so I suspect the extra foot pressure involved is related, but honestly it wasn\\'t much extra. When I had the valve on the part store bench the guy removed the rubber cap and pushed in just one time on the spring loaded cylinder and it popped right back into place which seemed perfectly fine to me but I now suspect it may be related. Thanks for any comments or advice! Vanric

I now have knowledge that it is in fact called a proportioning valve and it is not uncommon for them to fail and it is a Ford dealer only item which costs $225 for this particular one.
Due to the likelihood and the potenial extreme consequences of a future failure, I decided to pay the outrageous price for a new proportioning valve in order to be safe instead of even considering a used part even if one could be found.
I\\'ve always felt adamant about having brake systems in excellent condition and have always replaced all the other components long before they expire, but for some reason never felt a need to be concerned about steel brake lines or these proportioning valves and have never had a brake fluid leak in over 40 years of driving, but after this proportioning valve failure nearly causing a major disaster in addition to having a steel brake line burst due to corrosion on both my 88 Ranger and this 86 Crown Vic in just a matter of the last few weeks... I will from now on never feel safe without knowing they are reasonably fresh parts.
I encourage everyone to learn from my experience and think of it as a responsibility to yourself, your passengers and everyone in your path to replace the steel lines and proportioning valves within a reasonable time frame because lives really do depend on it !!!
BTW: double flaring is a must for steel lines and that requires special tools and learn-able skills.