Loosing RPMs when i press the brake **Update**

[Mustang Boy]

today on the way home i had the problem of my car loosing RPMs whenever i touched the brake not only when i slowed down but even pressing it ever so gently for just the brake lights to come on there was no problem when i was letting it coast nothing happened only when i hit the brake did the RPMs start to go down

 

[wallaka]

Sounds like your master cylinder might be leaky.

 

[Mustang Boy]

how would that cause my RPMs to drop to almost the level of stalling without me having to hit the gas to keep it running

 

[wallaka]

Oops, I meant the brake booster . Anyhow, it uses engine vacuum to work and if the diaphragm inside has a hole worn in it, it's a big vacuum leak when you touch the brakes.

 

[Mustang Boy]

i dont have a brake booster i still got my stock manual brakes in my 66 mustang

 

[wallaka]

Hmmm. Strange then. I have no idea.

 

[matt1967]

sounds like a torque converter issue, remaining locked or not slipping enough.

 

[wallaka]

I guess it only does it in gear?

 

[Mustang Boy]

i have a manual tranny and it does it whether its in gear with the clutch pressed or out of gear w/ or w/o the clutch pressed

 

[addo]

Or, an electrical short in the brake lines, causing ignition weakness...

 

[Mercury Mike]

I think Addo might've hit it on the head.

 

[matt1967]

or a weak alternator. might be so weak the brake lights are putting such a load on the battery that it starts to run bad. Go to Harbor Freight and get a $3.99 7 function digital multimeter, and set it to DCV 20. cut the prod's off the wires and make them longer, then run the neg to the battery and the red to the post on the coil for power, normal conditions this should read around 6.5 V, run the leads through 1 of the firewall grommets and drive the car around, hit the brakes, if the voltage stays at least 6v than it rules out any electrical problem. this would test for Addo's suggestion also.

 

[Mustang Boy]

well the alt is only 3 weeks old and it all of the other electrical things in my car seem to be working fine even my radio which i had pumpin pretty loud on for most of the way home

 

[matt1967]

well the alt is only 3 weeks old and it all of the other electrical things in my car seem to be working fine even my radio which i had pumpin pretty loud on for most of the way home

did you buy the alt from Auto Zone or Advanced Auto? Don't assume new parts are good because there new. often there not. could be a voltage regulator problem, not sensing the extra load, the old external regulators are not solid state, and have moving parts, they go bad often.

 

[Mustang Boy]

i also got a new voltage regulator when i got the new alt and yes i did get it from a parts store and it was the second one i had bought that day cause the first was messed up and we checked the voltage when we switched it and it worked just right then and we even tried it with the headlights on which i would think pulled more power than the brake lights

 

[matt1967]

i also got a new voltage regulator when i got the new alt and yes i did get it from a parts store and it was the second one i had bought that day cause the first was messed up and we checked the voltage when we switched it and it worked just right then and we even tried it with the headlights on which i would think pulled more power than the brake lights

yes, but the left headlight grounds through it which may affect the way the regulator acts on it, not to mention it may have arleady went bad, what brand made it, the 1 I put on my '67 Falcon is made by Standard Motor Products made for Carquest, and works like a charm . do you have a DVOM tester? if so just set it to check DCV and put the black prong to a source of ground and then use the red on the power source on the coil, have someone get in and start it, then push on the brakes, voltage should not drop below 6v

 

[fordconvert]

Id go after every ground I can find. Is the ground strap between the engine and body still in place?

Maybe I missed it but do you have a tach (factory or other?) or are you just hearing the rpm drop? What kind of ignition system you running?

How about a broken motor mount messing with the throttle? That certainly could happen with the stock linkage so if it could happen on yours would depend on how you did the linkage for the 2bbl.

So let me get this clear, you are driving at 35 mph in 3rd gear on level ground and without changing your throttle setting you push the brake pedal slightly with your left foot and the rpms drop?

 

[Mustang Boy]

i have a pertronix ignition with a blaster 2 coil and i dont have a engine to chassis ground that i know of i have an aftermarket tach from summit racing and doin the last thing would affect the rpm cause i have a manual and but when coasting letting it idle it idles fine at 1400 RPMs but as soon as i start to hit the brake my RPMs start dropping and i can hear it and see it on the tach

 

[Lazy JW]

You definitely need to check for voltage drop at the coil as suggested above.
Joe

 

[Mustang Boy]

ok ill try all of this today