"72 Cadillac Brake booster issue

[Scot Minesinger]

Glen,

I have seen a muffler type device in line between carb vacuum and booster on 1976 Cadillac RWD.  Did not know what it was for, but I was charged with replacing vacuum hoses, so left it as is and did not question.  I will be starting to work on a 72 Eldorado and while driving it a short distance did not notice the hissing sound, but will listen for it now.

[Bill Young]

Thank You Gavin, No, mine makes the loud hissing all the time when the brake pedal is applied. it starts quietly when you start pushing on the pedal and increases in volume as you increase pedal pressure till the loudest is at full pedal pressure and then decreases again as you release pedal pressure and stops when you remove your foot from the Brake pedal completely. I did not think to check further up inside the dash above the pedal to see if a hose connection or device accorded with the Cruise Control function could be causing the sound. Also though in any case I don't regret replacing the Booster and Master Cylinder with the new rebuilt Cardone unit as the Booster that was in the Car was original and as I use this Car as my daily driver I want to be double sure it is super safe not only for My protection but for the protection of others on the road and their Families cause if I run into a small foreign car with this thing they will need every air bag and crumple zone they have and I don't want that.

[Bill Young]

Scot, if this is occuring you would not have to listen for this, it is so loud that raising the volume level on the Radio too loud can not drown it out. Again I am going to remove the dash lower panel below the streeing column and check any items that interface with the brake pedal there.

[Bill Young]

I found the source of the hissing sound. I removed the lower dash panel and there is a black plastic switch that is activated when you press on the brake pedal that has a large healthy vacuum hose attached to the back side of it. I assume it has to do with releasing the Cruise Control when You apply the brake pedal. It is worn out internally aparently and so I disconnected the hose and screwed a bolt into it , problem solved as the Cruise Control does not work anyway and I have larger fish to fry than CC not working.

[TJ Hopland]

That is indeed part of the cruise control.  Its the 'emergency' vent / kill.  There is also an electrical switch in the brake switch that opens the vent solenoid so that one is just to make sure.

There should only be vacuum there when the cruise is operating.  Sounds like your cruise has some serious issues.  Since you say its not working and you have bigger issues I would disconnect and plug the vacuum feed to the gearbox dodad on the firewall so it does not cause a vacuum leak or worse yet suddenly engage the cruise full blast.   The control unit is the thing with the speedometer cables going to it.  Should not be too hard to figure out which is the supply line, just pull it off and cap/plug the line till you get around to fixing it. 

[Bill Young]

Good point, thanks! Is it possible for the cruise to engage with the switch on the dash in the off position?

[TJ Hopland]

Its not supposed to be able to engage but its also not supposed to have any vacuum at that hose on the brake pedal when its not running.

If you look at the vacuum diagram you will likely see that the brake pedal hose is teed into the line that goes to the vacuum motor that actually tugs on the throttle.  The idea is that if you step on the brake it unplugs that vacuum line so no matter what the electrics are doing you will 'dump' the vacuum in the servo and release the throttle. 

[Bill Young]

My God, after listening to your explanation of that I feel too stupid to own this Car.

[Scot Minesinger]

Bill,

Glad I had something to do with the resolution.  Sorry you changed out your booster for nothing.  I just changed mine on my 1970 and it fixed the problem.

[mgbeda]

That is weird that there was vacuum in that line with the cruise off.  I wonder if somebody mis-plumbed the vacuum lines somehow.

I would also disconnect the vacuum line that runs to the cruise actuator.  That's the big (about 5" diameter) diaphragm looking thing mounted towards the back of the engine on the top left.  If you did have a vacuum leak in the emergency bypass switch (on the brake pedal) and now you fixed that there could now be MORE vacuum trying to pull on that actuator.

-mB

[The Tassie Devil(le)]

G'day Mike,

These Vacuum operated Cruise Controls have a vacuum "switch" at the brake pedal as well as the electric one.   This way, there is a back-up turn off in case the electrics, or the vacuum fail.

Bruce.

[mgbeda]

G'day Bruce.

I knew about the vacuum switch, but isn't it between the cruise transducer and the cruise actuator?  If so there should be no vacuum there when the cruise is off.  If there is vacuum there with the cruise off it could pull on the actuator and cause the engine to rev, even with the cruise off, and now that the brake vacuum switch has been plugged, even with the brake on. 

Then I thought it was weird that the cruise would fail such that it is always on (Plenty of cases of losing vacuum, but here's a case of vacuum present when it shouldn't be).  So I wondered if maybe someone saw one of those nice fat vacuum hoses that attach in the actuator circuit and went and hooked them up to a source of engine vacuum.

-mB

[Bill Young]

Gavin, I am not for sure what is causing Your hiss. In My cars case it turns out that it was the Cruise Control switch that acts on the brake pedal to stop the activation of the Cruise that was the problem, it hissed all the time when I pressed the brake pedal not just pressing but holding it pressed also. So I plugged that hose to stop the problem as my cruise does not work and I don't have the money to fix it. Also I recently replaced both my master cylinder and booster and I do not regret doing that as I drive my car every day to work and I want safety for me and anybody else on the road around me.
Bill

[Scot Minesinger]

Bill,

The cruise may not be expensive to fix.  Start with a new brake vacuum switch (probably good used).  Then it is likely either the vacuum modulator that tugs on the throttle, bad vacuum hoses, or the transducer itself.  The transducers are sold re-built, the vacuum actuator could be re-built or bought used on e-bay.  It may be something else, but those seem to be the primary sources of failure on these 70-76 Cadillac (all GM?) cruise controls.

Understand what you mean, in our area there is no using the cruise until you leave the metro area (travel 40 miles to 80 miles) I live in, which is infrequent in my 1970 Cadillac.

[Bill Young]

Thank You for the tip Scot. When I can catch my breath financially I will attempt that. It would be exciting to see things come back to life, we'll see.