Vacuum Brake Booster Replacement
Thread Starter
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Vacuum Brake Booster Replacement
"I mean, how hard can this be"? -- The introduction to a really old (1980s vintage) Apple Macintosh programming article.
I had been having rough idle problems for about a month with no hints at what was wrong.
I had a month-old O2 sensor fail, which seemed related but didn't fix the problem.
One day as I was backing out of the garage I pressed on the brake pedal and it was VERY hard.
I figured the vacuum brake booster diaphragm had ruptured.
I plugged the vacuum line going from the plenum to the booster and the idle immediately returned to normal.
I thought the diaphragm had a small tear that finally blew out when the vacuum hit it during startup.
It's only two nuts on the inside...
...And two nuts on the master cylinder side. I was surprised that it came with a new vacuum check valve.
I unbolted the master cylinder first because I figured it would be easier with the booster still attached to the firewall. 13mm socket on both nuts.
The brake lines are flexible enough to move the master cylinder out of the way.
I did this about 15 years ago. That time I only replaced the diaphragm, which had a 4" tear in it. I don't think you can buy just the diaphragm any more.
That's fire extinguisher powder (more on that later) on the top of the master cylinder caps. It's really hard to clean up that stuff. I tried, but this was as good as it got.
Underneath the dash. The hidden nut is pretty hard to get to. A long extension with a flexible joint was helpful. 13mm nuts again.
The rod is held on the brake pedal lever pin with a spring clip, not a cotter pin.
The vacuum hose on the left comes from the cruise control servo and disengages it when the brake pedal is pressed.
I had a fire several months previously and the bottom of the brake booster got damaged. It didn't seem to affect the functionality.
You can see the diaphragm though the holes.
The booster disassembled. 13mm nuts again.
The diaphragm is mounted on the center section. It's the part with the powder on it (probably mold release).
The spring on the right is really strong and it's what makes the brake pedal so hard to push when the diaphragm fails.
It turned out that the diaphragm was fine. It was the cracked case that caused the problem. Looks like that area got a little toasted in the fire.
Overall about a 2 hour job, with lots of obsessive cleaning along the way.
I had been having rough idle problems for about a month with no hints at what was wrong.
I had a month-old O2 sensor fail, which seemed related but didn't fix the problem.
One day as I was backing out of the garage I pressed on the brake pedal and it was VERY hard.
I figured the vacuum brake booster diaphragm had ruptured.
I plugged the vacuum line going from the plenum to the booster and the idle immediately returned to normal.
I thought the diaphragm had a small tear that finally blew out when the vacuum hit it during startup.
It's only two nuts on the inside...
...And two nuts on the master cylinder side. I was surprised that it came with a new vacuum check valve.
I unbolted the master cylinder first because I figured it would be easier with the booster still attached to the firewall. 13mm socket on both nuts.
The brake lines are flexible enough to move the master cylinder out of the way.
I did this about 15 years ago. That time I only replaced the diaphragm, which had a 4" tear in it. I don't think you can buy just the diaphragm any more.
That's fire extinguisher powder (more on that later) on the top of the master cylinder caps. It's really hard to clean up that stuff. I tried, but this was as good as it got.
Underneath the dash. The hidden nut is pretty hard to get to. A long extension with a flexible joint was helpful. 13mm nuts again.
The rod is held on the brake pedal lever pin with a spring clip, not a cotter pin.
The vacuum hose on the left comes from the cruise control servo and disengages it when the brake pedal is pressed.
I had a fire several months previously and the bottom of the brake booster got damaged. It didn't seem to affect the functionality.
You can see the diaphragm though the holes.
The booster disassembled. 13mm nuts again.
The diaphragm is mounted on the center section. It's the part with the powder on it (probably mold release).
The spring on the right is really strong and it's what makes the brake pedal so hard to push when the diaphragm fails.
It turned out that the diaphragm was fine. It was the cracked case that caused the problem. Looks like that area got a little toasted in the fire.
Overall about a 2 hour job, with lots of obsessive cleaning along the way.
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Joined: Oct 2003
Posts: 94
Likes: 1
From: Chattanooga TN
Wow, what caused the fire? That stays in the back of my mind as I tune my car in my garage. I'll be sure and keep the can much closer!
I also had to change my brake booster some time back. I had to get one from advance auto or something, and it was universal fit. It was metal this time, which was fine except for adding a slight amount of weight, which I assume was the orginal reason for the plastic. I also had to paint it. The good news, the rest of my car isn't stock, and the part was only about $20 with core return.
I also had to change my brake booster some time back. I had to get one from advance auto or something, and it was universal fit. It was metal this time, which was fine except for adding a slight amount of weight, which I assume was the orginal reason for the plastic. I also had to paint it. The good news, the rest of my car isn't stock, and the part was only about $20 with core return.
Thread Starter
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
I actually have had two fires in this car. I carry a fire extinguisher from the days I autocrossed this car.
The first one was a ruptured fuel pressure regulator diaphragm. The vacuum line to the FPR blew off and fuel sprayed all over the engine. It was running off the front and rear of the intake manifold and the plenum had fuel in it. The fuel caught fire when it hit the driver's side catalytic converter. When I opened the hood flames were coming out from under the plenum in front of the distributor.
A friend of mine gave me the 700r4 out of his '89 when he put in a 6 speed manual trans. I rebuilt it and put it in my car. I had a transmission line pressure gauge hooked up and was driving around monitoring the pressure. I had the hose tied up but it was touching the exhaust shield near cylinder #7. I thought that would be OK, but apparently not. The hose burst and sprayed trans fluid all over the driver's side of the engine compartment. It caught fire when it hit the driver's side cat. I saw flames coming out the rear of the car and pulled into a parking lot. When I popped the hood the whole left side of the engine was on fire. Flames were coming out of the holes in the rear of the alternator. There was a puddle of burning trans fluid under the car. Parts of my upper plastic wheel well melted.
I never thought I would need a fire extinguisher, but I'm really glad I had one, both times.
The first one was a ruptured fuel pressure regulator diaphragm. The vacuum line to the FPR blew off and fuel sprayed all over the engine. It was running off the front and rear of the intake manifold and the plenum had fuel in it. The fuel caught fire when it hit the driver's side catalytic converter. When I opened the hood flames were coming out from under the plenum in front of the distributor.
A friend of mine gave me the 700r4 out of his '89 when he put in a 6 speed manual trans. I rebuilt it and put it in my car. I had a transmission line pressure gauge hooked up and was driving around monitoring the pressure. I had the hose tied up but it was touching the exhaust shield near cylinder #7. I thought that would be OK, but apparently not. The hose burst and sprayed trans fluid all over the driver's side of the engine compartment. It caught fire when it hit the driver's side cat. I saw flames coming out the rear of the car and pulled into a parking lot. When I popped the hood the whole left side of the engine was on fire. Flames were coming out of the holes in the rear of the alternator. There was a puddle of burning trans fluid under the car. Parts of my upper plastic wheel well melted.
I never thought I would need a fire extinguisher, but I'm really glad I had one, both times.
