Booster reading
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Booster reading
How’s it going
trying to diagnose an issue w brake booster on a 65 f350 dually firewall mount.High and stiff pedal. My vacuum reading to booster including check valve is 22 which seems ok. Just wanted to confirm that reading. Thanks in advance
Dennis
trying to diagnose an issue w brake booster on a 65 f350 dually firewall mount.High and stiff pedal. My vacuum reading to booster including check valve is 22 which seems ok. Just wanted to confirm that reading. Thanks in advance
Dennis
More Turbo
Joined: Oct 2009
Posts: 596
Likes: 58
From: West Plains, MO
22" of vacuum is great! But does it drop momentarily when the pedal is depressed (e.g. maybe the internal booster valve is not opening, which is why you have such a stiff pedal with a good vacuum)? If you're idling at (say) 18", does the gauge drop from 22 to 18? If it stays on 22 the vacuum is not being "used"... 
One test for this used to be, shut the engine off, and watch the brake pedal carefully when restarting. There should be a little twitch downward. I think, it's been a long time.
One test for this used to be, shut the engine off, and watch the brake pedal carefully when restarting. There should be a little twitch downward. I think, it's been a long time.
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Freshman User
Joined: May 2021
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22" of vacuum is great! But does it drop momentarily when the pedal is depressed (e.g. maybe the internal booster valve is not opening, which is why you have such a stiff pedal with a good vacuum)? If you're idling at (say) 18", does the gauge drop from 22 to 18? If it stays on 22 the vacuum is not being "used"...
One test for this used to be, shut the engine off, and watch the brake pedal carefully when restarting. There should be a little twitch downward. I think, it's been a long time.
One test for this used to be, shut the engine off, and watch the brake pedal carefully when restarting. There should be a little twitch downward. I think, it's been a long time.
Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
That's all good info but how well does it stop? I would think that the pedal travel wouldn't be very much with 4 wheel drum brakes that are adjusted the way you have described.
If you have to apply a very high amount of pressure to stop, more that likely you will need to replace your booster.
Don't overlook anything like blockages in the hose or the hose collapsing on the inside.
If you have to apply a very high amount of pressure to stop, more that likely you will need to replace your booster.
Don't overlook anything like blockages in the hose or the hose collapsing on the inside.
Lead Gopher
Joined: Aug 2012
Posts: 10,014
Likes: 1,565
From: Seattle
Most of the boosters I've worked on are very simple. Here's an original style brake booster like used on the F100 and F250 in 1965 and 1966. C5TZ-2005-C. This is a bit smaller diameter than the F350 used, of course, but I believe yours would be single diaphragm too.
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
Fleet Mechanic
Joined: Jun 2019
Posts: 1,898
Likes: 234
From: Southern California
Most of the boosters I've worked on are very simple. Here's an original style brake booster like used on the F100 and F250 in 1965 and 1966. C5TZ-2005-C. This is a bit smaller diameter than the F350 used, of course, but I believe yours would be single diaphragm too.
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
OP Dennis, and Doc Charles. To test booster, turn off engine, pump pedal several times to exhaust all reserve vacuum, put your foot on the brake pedal and start the truck. The pedal should fall away slightly when the motor starts and pulls a vacuum. If it doesn't you have a defective booster, or no vacuum to the booster. period.
Lead Gopher
Joined: Aug 2012
Posts: 10,014
Likes: 1,565
From: Seattle
Not advising anyone to go to this depth but just as an FYI here's one on eBay. I found out about it and the procedure years ago from my 1971 Pontiac Firebird manual.
Kent-Moore J-9504 Brake Booster Spanner 1962-68 | eBay
Kent-Moore J-9504 Brake Booster Spanner 1962-68 | eBay
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Lead Gopher
Joined: Aug 2012
Posts: 10,014
Likes: 1,565
From: Seattle
Here's another which given shipping costs of the other this one would be cheaper for me.
Kent Moore J-9504 Vacuum Brake Booster Spanner Wrench Tool | eBay
Kent Moore J-9504 Vacuum Brake Booster Spanner Wrench Tool | eBay
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Lead Gopher
Joined: Aug 2012
Posts: 10,014
Likes: 1,565
From: Seattle
I will say that I do dig into things that some people wouldn't. My Uncle told me something years ago when I was about 12 or 13 that stuck with me to this day. I was struggling with removing the crankshaft damper on a Ford 534 V8 in an F700 or F750 dump truck and he came over and ask if I wanted some advice. I said "sure" and it was then that he simply looked at me and said "What would you do if I were not here?" and then he went back to what he was doing. Really made me realize I just had to put my mind to it. Mind over matter. Or, if you don't mind it doesn't matter...
Thread Starter
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Freshman User
Joined: May 2021
Posts: 25
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Most of the boosters I've worked on are very simple. Here's an original style brake booster like used on the F100 and F250 in 1965 and 1966. C5TZ-2005-C. This is a bit smaller diameter than the F350 used, of course, but I believe yours would be single diaphragm too.
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
You can disassemble them with a brake booster spanner. The blue one is 36" long. In this case the two halves capture the outer edge of the diaphragm which makes the seal separating front from back. The check valve is there to try to preserve the vacuum on the front half when the engine stops otherwise it should be open to allow the vacuum the enter the chamber. That places a draw on the diaphragm at all times when running and only when you apply foot pressure to the pedal does open the rear intake to override the internal spring pressure holding the diaphragm back. A foam filter allows replacement air to be drawn from the cab where the push rod enters the rear half to allow the diaphragm to move forward on demand. Not a lot that can go wrong. Depending on where you draw vacuum from the check valve can get sticky from oils being drawn in if you have some blowby. The diaphragm can tear due to age. The foam filters deteriorate - I've been using surgical grade foam to cut new filters by hand and it will hold up for a lot longer than the original stuff. Are you still using the original single chamber master cylinder or has it been swapped to a dual chamber master?
so check this out… I found out that after I hooked the vacuum line back to the booster after adjusting the idle/air, it ran crappier. So my brother gave me a pro tip. I sprayed starter fluid around anywhere there could be a leak in the booster and noticed a change in the engine idle. Hence the booster is leaking. The o ring where master cylinder attaches was a little rough as well as where it seats to that rubber part of the diaphragm on the outer side. Taking on getting that sealed up tomorrow . Diaphragm felt ok and dry of brake fluid so that was good at least.
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