Festiva-- How to adjust brake booster pushrod to master cylndr-- is there a locknut?

[steve s]

Hi all. My brake pedal has to go pretty far down before the brakes engage and I suspect the pushrod from the brake booster is not inserted far enough into the master cylinder. The Ford shop manual gives detailed measurements as to how far the pushrod should be inserted into the master cylinder and says something about loosening a locknut to adjust it. I see no evidence of a locknut but I can't turn the pushrod-- I wonder if some previous owner or mechanic just screwed the pushrod in toward the booster as far as it go till it came to a hard stop? Should I just twist really hard on it to loosen it? Or am I overlooking a locknut somewhere that I can loosen?

Thanks for any hints--

Steve

[steve s]

Finally got it turning-- it turns very hard-- no evidence of a locknut; maybe some cars were equipped with a booster from some other vendor that did have a locknut on the pushrod but I don't think this one has one

[bravekozak]

Yes there is a jam nut on the pushrod at the clevis.l
First, loosen the jam nut.
Adjust the pushrod distance by rotating the rod in or out.
Then, tighten the jam nut.

[steve s]

Thanks-- I'll attach a photo or a link to a photo of the pushrod coming out of the booster-- I'm not sure where the clevis is that you are referring too-- is the jam nut the knurled area near the knife blade? Is that knurled thing actually two pieces, so the front one (front is down in the photo) can be backed away from the back one to let the pushrod end turn? Or is the whole knurled area the jam nut? Or am I looking in the wrong place? Thanks! Steve

[steve s]

A clarification-- What i want to adjust is the actual clearance between the front end of the pushrod that comes out of the front of the booster, and the piston pocket on the master cylinder that that pushrod goes into. The booster is still mounted on the car, so the back end of the booster is not exposed, but I've separated the master cylinder from the booster. I see from the shop manual that there is also a pushrod coming out of the back end of the booster and that pushrod has a clevis on its back end and I can see that adjusting that pushrod would affect the height of the brake pedal but as I understand it, that adjustment doesn't really affect the clearance between the front end of the front pushrod and the piston pocket in the master cylinder, which I understand is important for proper braking performance. The shop manual gives a detailed measurement for what this clearance should be -- .016 to .024 inches in the no-vacuum case-- and the clearance in my car is much larger than that. So it's more than just a pedal height adjustment that I think I'm trying to accomplish here-- but I can't seem to adjust the length of this front pushrod. I don't think adjusting the pushrod on the back end of the brake booster is really going to address my issue.

Is there a locknut in my picture and I'm just not seeing it?

[bravekozak]

Hold everything!
Push rod length normally only needs to be adjusted when the master cylinder is replaced. Proper adjustment is critical because if the pushrod is adjusted too long, the brakes will drag. If the pushrod is adjusted too short, the brake pedal will be low. Master Cylinder gauge T87C-2500-A is required for accurate measurement and adjustment of pushrod length.

You simply push the plunger of the gauge against the primary piston of the master cylinder and tighten the set screw.
You then invert the gauge over the brake booster push rod and measurer the clearance. Adjust the pushrod for .019" clearance (with no vacuum in the brake booster diaphragm) by loosening the locknut, adjusting and then tightening the locknut back up again. Then re-install the master cylinder.
With vacuum, you will probably end up with about .005" of clearance.

[WmWatt]

Um, can't the height of the brake pedal be adjusted instead?

[steve s]

Re bravekozak-- thanks for the detailed notes-- that's exactly what we want to do-- we had reason to believe the pushrod was adjusted too short-- we made own version of the gauge -- upon separating the master cylinder from the booster it was immediately obvious that the pushrod was adjusted way too short. However, on the the 3 different units we've looked at (including the one in the picture) it appears that there is no locknut-- you just grip the knurled part of the rod with a vice grips, put a box-end hex wrench on the hex part of the shiny pushrod end, and turn. It turns hard-- maybe some kind of threadlocker has been applied-- but there is no actual lock nut as far as we can see. Let me know if you think we're missing something here-- thanks.

Steve

Hold everything!
Push rod length normally only needs to be adjusted when the master cylinder is replaced. Proper adjustment is critical because if the pushrod is adjusted too long, the brakes will drag. If the pushrod is adjusted too short, the brake pedal will be low. Master Cylinder gauge T87C-2500-A is required for accurate measurement and adjustment of pushrod length.

You simply push the plunger of the gauge against the primary piston of the master cylinder and tighten the set screw.
You then invert the gauge over the brake booster push rod and measurer the clearance. Adjust the pushrod for .019" clearance (with no vacuum in the brake booster diaphragm) by loosening the locknut, adjusting and then tightening the locknut back up again. Then re-install the master cylinder.
With vacuum, you will probably end up with about .005" of clearance.

[steve s]

Just got a brand new booster where things are not all corroded and stuck-- still trying to figure out if that knurled thing on the pushrod is a locknut or just part of the shaft-- if it's a locknut it's not turning easy even in the brand-new one--

If that knurled thing is in fact a locknut, when you put a vice grip on it to turn it, where do you put your other wrench to get opposite leverage? You can't clamp down with another vice grip onto the smooth shaft further aft (back) because that surface has to be undamaged to slide through the dust seal. Do you just put a box-end wrench on the hex surface of the shiny pushrod end and turn clockwise on that wrench (as viewed from the front) at the same time as you turn counterclockwise on your vice grips clamped down on the knurled surface? That's the direction that would TIGHTEN the shiny push rod end if no lock nut were present-- but if there is a lock nut (if the knurled surface is a lock nut) maybe this gives the leverage to break the lock nut free?

Still a little lost at the moment--

S

Re bravekozak-- thanks for the detailed notes-- that's exactly what we want to do-- we had reason to believe the pushrod was adjusted too short-- we made own version of the gauge -- upon separating the master cylinder from the booster it was immediately obvious that the pushrod was adjusted way too short. However, on the the 3 different units we've looked at (including the one in the picture) it appears that there is no locknut-- you just grip the knurled part of the rod with a vice grips, put a box-end hex wrench on the hex part of the shiny pushrod end, and turn. It turns hard-- maybe some kind of threadlocker has been applied-- but there is no actual lock nut as far as we can see. Let me know if you think we're missing something here-- thanks.

Steve

[steve s]

Addendum-- we now have clear evidence that on the old unit that just came off the car, the knurled thing is NOT a lock nut but rather is just a feature molded or forged onto the pushrod that sticks out of the booster -- we see no evidence of a locknut on the pushrod to the master cylinder on any of the three booster units we've been looking at -- S

[bravekozak]

The jamnut is on the brake pedal clevis.
Everything about brakes starts at page 28.