Modulator valve and servo seal/gasket

Good afternoon all,

With winter coming to an end my son and I are wanting to dry up a couple of transmission leaks on the C4. The car is a 76' (w/ 302), we will try to confirm the transmission by looking at the tag or stamp (though I didn't notice a tag last night). The car shifts and drives quite well, but leaks enough that you have to add often. The apparent primary leak appears to be from the driver's side servo. All the bolts are tight, but I'm relatively certain that it is leaking past the cover plate. Neither the piston seal nor cover plate gasket is expensive, I assume I should renew both.

Secondly, there was transmission fluid in the rubber hose that connects to the vacuum modulator. This transmission appears to have the vac modulator that screws in, but there was no nipple on the other end. The short piece of plastic hose from the hard line above just slides inside the modulator (pulls out very easily). Obviously the transmission fluid is coming through the modulator, so I assume it needs replaced as well. The few searches I completed here typically included shift problems related to the vac modulator, but the car shifts fine. Thank you in advance for any help, my 16 year old son is excited to get the old gal turned into a daily driver.

Chris & Gabe

 

Vacuum modulators more often leak through the diaphragm than give shifting issues. Even a small leak turns into a big one when engine is running, vacuum sucks fluid into intake.

 

there is also a...shifter shaft seal... on the drivers side. it has a tendency to leak also.

 

Thank you fellas. We will remove the old v/m and get one that matches, I assume jacking it up limits oil loss?

We will change the servo seals as well. Is there access to a PDF service manual on this site?

TIA,

Chris & Gabe

 

If you pull that servo piston, you'll be dropping the pan, and valve body. At that point, you might as well replace the shifter seal also.

 

Thanks Will,

I was picturing a little more room, with that in mind, we may just start with the cover gasket and modulator and see what happens. Thank you.

Chris & Gabe

 

It's not just a matter of having room. If the piston comes out, the strut will be free to fall out, then you'll be having to pull the valve body to get it back in position.

 

Was common for the later '60s C4 to break the intermediate band, allowing the struts to fall onto valve body. This usually limited travel of the manual valve, causing a jamb when trying to shift reverse(or park). Results were transmission had 1st & 3rd only. In drive it'd start out normally, freewheel trying to shift to 2nd, then grab 3rd. I knew someone that drove a '69 Torino for several months in that condition. He finally sold it to me as a parts car after timing chain jumped.

 

if you drop the VB watch for the 2-3 balls that will fall out . they have a certain place they need to be when you replace the VB.

 

Thanks guys,

For now we are just going to renew the extension housing gasket and see if that seals us up.

This transmission currently has the push-in yellow stripe vacuum modulator. Obviously we need to stay with the push-in style, but are the adjustable modulators better (different color stripes)?? What do most folks replace with??

TIA,

Chris

 

A little search offered this info.

poke here

It would seem the green or yellow (both likely adjustable) would work. With a 302, I went ahead and got a yellow stripe coming. Thx.

 

Hey all,

We got the vacuum modulator changed but the transmission is acting up, lacks forward gears. With so little room to maneuver, I placed the "pin" in the end of the new vacuum modulator and slid it into place (push-in style). I didn't have to fight it and it seemed to slide in ok. The adjustment screw that goes in the vacuum modulator had come out during shipping, I put it back in and put two turns on it, just enough to feel it wouldn't work its' way out to easily. We put the keeper back on and then renewed the rubber lines. Since the hard steel vacuum line is quite difficult to get to, we disconnected the transmission dipstick and hard line support on the rear of the engine to allow movement of the hard line. Lowered the hard line down to allow access and got the new hose on. In the process, the transmission dipstick housing came out.

I love working between the firewall and rear of the engine (NOT!). I noticed the transmission dipstick pipe was not seating clear to the shoulder, upon inspection with my finger into the transmission "receiving hole", I felt what I would describe as a rounded ball bearing that is keeping the housing from inserting clear to the shoulder. It wasn't loose in any way and didn't want to move at all. The transmission had been driving ok, we were sealing a couple of leaks that needed attention.

We also placed a new gasket on the servo cover plate to address a leak there as well.

Could the vacuum modulator pin be mis-aligned and contributing to the issue?

What is likely keeping the dipstick housing from seating properly and how to correct?

Transmission has sufficient Type F fluid that is still bright red. All help appreciated, thank you.

Chris & Gabe

 

Been my experience they'll operate with the pin missing but have early, slipping shifts.

 

I know the pin is there, I wasn't sure of exactly where/what it landed in/on the inside and if it could miss the mark.

Chris & Gabe