1988 SR5 pickup, 2WD, 470K

My questionareis: Why will the brake pedal rise soon after adjustment? And why can't I adjust the brake booster rod to the H tool? Might there be an issue with the vacuum system, that increases vacuum after driving awhile?

I bled my brakes after replacing the front pads. The brake pedal felt a bit low, so I adjusted the brake booster rod toward the master cylinder.
I took a test ride around the neighborhood and returned home to clean the windshield before a longer and faster test ride.
As I headed down the local "heart attack hill," the truck did not accelerate. When I reached the bottom on the hill, the truck seemed to drag during shifting.
As I started out from a stop sign, I could smell the brakes. I limped the mile home.

I measured the master cylinder and BB rod with my H tool. The rod was about 0.030" short of the length indicated by H tool. I took out 2/3 of the previous adjustment and took another ride. Now the difference is about 0.050."

Pedal height was good, and the brakes worked well, but the rod length did not match the H tool.

 

If the booster rod is too far out it will cause the master cylinder to not release pressure. The brakes would be fine at first because there would be very little drag on the pads, but as you drive the brake components will heat up and expand slightly. Since the fluid cannot be pushed back into the master cylinder reservoir, it basically applies more pressure to the brakes, heating them up more, and the cycle continues.

Maybe the H tool is defective? If you place it on a flat surface, the rod in the middle should not be longer or shorter than the other legs of the H on both sides if that makes sense. The rod could be a few thousandths longer to account for a small gap between the rod and the master cylinder piston, which would be fine. The other possibility is that the booster push rod and the H tool rod are significantly different thicknesses enough to hit the domed shaped master cylinder piston hole in different places.

If something is wrong with the booster, I am thinking there is a chance it could be applying slight pressure to the master cylinder and holds just enough so .050" extra adjustment makes up for it. I have personally never experienced that, but knowing how a brake booster works it seems plausible. I would think that if that is the case, then you could see the pedal sink slightly once the engine is started, assuming all vacuum was release beforehand.

 

To the_supernerd I thank you for your explanation of the interaction of the booster rod and the brake cylinder tunnel. It was very clear.
The H tool is OK. the pin is about 0.005" shorter than the legs of the H.

When I drove to the hardware store today, I stopped three times to turn the rod away from the master cylinder. (I can reach under the dashboard and turn the rod into the clevis with pliars without removing the brake cylinder. Not real scientifical, but the H tool doesn't give me reliable info.) I turn the rod about half of a rotation each time.

When the brakes dragged, I felt the wheels for heat: only the front wheels were warm.
When I returned home without further event, I measured the pin length: there is a 0.118" gap from the pin of the H tool to the BB rod.

Maybe the head of the BB rod is too wide.

When the brakes drag, the truck shudders and chatters as I begin to drive. Previously I thought this was a clutch problem that arose after driving 15-20 miles.

 

I might be a bit confused, but the H tool is for adjusting the rod between the brake booster and master cylinder under the hood. The rod under the dash is part of the booster that connects to the pedal. Typically you adjust that so the brake pedal has just a small amount of free play before contacting the rod, and enough so the brake lights shut off when the pedal is not being pressed. The free play you feel there should be a very loose feeling on the pedal, before it starts to push brake fluid. There should not be much play there unless something is significantly worn.

If you are chasing down excess pedal travel, first make sure to bleed all 4 wheels plus the load sensing proportioning valve. Another big source of play is rear brake pads that need adjusting. If the problem started after bleeding brakes, then there is either air in the lines or there was corrosion in the master cylinder and pushing the pedal to the floor while bleeding finished it off. Typically if there is air in the lines then the pedal height will be inconsistent and will rise as you pump it. If you push the pedal real hard and it sinks, that usually points to a master cylinder or a big leak in the system somewhere.

 

Thanks. I was pondering that yesterday. The rod is easier to adjust under the dash, but it might not be optimal,
There is probably a neutral position of the rod in the clevis which I probably didn't consider when I installed the new brake booster nearly two years ago.
I think I bled the brakes OK. The pedal does not feel loose, which might not be right.

How can I adjust the pedal height without affecting master cylinder.

Tomorrow.

 

The rod that connects from the pedal to the booster, and the push rod that goes between the booster and master and two separate rods that need adjusting separately. I would start by adjusting the one under the dash by adjusting the pedal all the way up until there is just a bit of free play before it starts pushing on the clevis pin. That way you know the booster is fully disengaged when the pedal is up. Then, go to the rod between the master cylinder and the booster and use the H tool to adjust it. Once that is done, if there is still play I would first check and adjust the rear drum brakes.

If there is still significant play after adjusting everything then the rest of the brake system needs to be checked. It could be a lot of things at that point, but I would start with proper adjustments first.

 

Today I will be aligning the clevis with the linkage to the brake pedal. When I removed the clevis pin yesterday, it was tight. With the pushrod turned all the way into the clevis, I have to push against the booster to align the holes. Not so good. I think I need to shorten the pushrod and to lengthen the thread on the pushrod. at least 1/4 inch. Then install the clevis pin without tension.

EDIT 4-14-26 afternoon. I removed the booster today and tried fitting all the pieces together. With with the clevis at the shortest position on the pushrod, and the booster hanging loose on he firewall, I was able to connect the pedal link to the clevis with the clevis pin: the booster body is about half of an inch from the firewall. There was stiff resistance to pushing the booster body closer to the firewall.

I think I need either a shorter clevis, a shorter pushrod, or a 1/2" spacer between the booster and firewall. I am looking.

 

If I remember correctly, there are two different thicknesses of brake booster spacers that came from the factory on these trucks. My 88 2wd pickup has the thicker aluminum one, possibly around an inch thick. Should be a cheap and easy junkyard item if there is one near you.

 

I have a spacer I saved from a 2WD 86 Xtra Cab. I'll have to make sure I still have it tomorrow.

 

VOILA.
i estimated the gap to be about half of an inch. I tried some washers that totaled 3/8." The clevis pin slipped in easily, and there was no preload on the booster when I put it in place by hand. I set the pin from the H tool. The brakes work well. I will adjust the pedal a bit higher tomorrow.
edit 4/17: I will fill the gap with automotive strength silicone.