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OK, here is my question. If you are driving along, and need to brake because you are coming upon a red light at an intersection, when you start to brake, is you pedal firm? Now, while you are sitting at the red light, does your brake pedal start out firm and then as you wait for the light to change, does the pedal sink? If it does, then I would say your power brake booster needs to be replaced.
OK, here is my question. If you are driving along, and need to brake because you are coming upon a red light at an intersection, when you start to brake, is you pedal firm? Now, while you are sitting at the red light, does your brake pedal start out firm and then as you wait for the light to change, does the pedal sink? If it does, then I would say your power brake booster needs to be replaced.
Pedal is not firm when starting to brake on the answer to the first question.
Second question, if my pedal is firm while at a stop, then yes, it does sink.
The hissing noise you hear when pressing the pedal is air being pulled into the brake booster as the diaphragm is moving - it’s totally normal. The seal on the cab side cracks and shrinks with age and as atmospheric pressure enters the booster the hissing sound happens. As long as the engine doesn’t start stumbling and running rough while the pedal is pressed (indicating the diaphragm is leaking) the booster is functionally fine.
Does your truck have the newer oval reservoir on the master cylinder or the older style rectangular reservoir?
Originally Posted by chem_man
Welcome to the Forum!
I too have a 99 Silverado.
OK, here is my question. If you are driving along, and need to brake because you are coming upon a red light at an intersection, when you start to brake, is you pedal firm? Now, while you are sitting at the red light, does your brake pedal start out firm and then as you wait for the light to change, does the pedal sink? If it does, then I would say your power brake booster needs to be replaced.
Just FYI this is not an indicator of a failed booster - it’s an indicator of a failing MC or air in the system. As long as the pedal is easy to press with the engine running, the booster works. A failing booster causes the pedal to get harder.
The hissing noise you hear when pressing the pedal is air being pulled into the brake booster as the diaphragm is moving - it’s totally normal. The seal on the cab side cracks and shrinks with age and as atmospheric pressure enters the booster the hissing sound happens. As long as the engine doesn’t start stumbling and running rough while the pedal is pressed (indicating the diaphragm is leaking) the booster is functionally fine.
OK so when I start the engine, the pedal goes to the floor. Engine never dies or stutters. EDIT: I do hear it drop down, and then when releasing the brakes, it returns to normal.
Does your truck have the newer oval reservoir on the master cylinder or the older style rectangular reservoir?
Rectangular
Just FYI this is not an indicator of a failed booster - it’s an indicator of a failing MC or air in the system. As long as the pedal is easy to press with the engine running, the booster works. A failing booster causes the pedal to get harder.
The master cylinder I replaced it with is brand new. I'm bleeding the brakes from the master cylinder. Just a note: Tonight I was just using it around the property to move some things a short distance. when I was coming down the hill, covering the brake with my foot, it felt stiff like it should. But as soon as I stopped, went to the floor.
I just don't know if I really need a booster or not.
EDIT: I was just outside trying to bleed the brakes again. The brake light is staying on, but periodically goes off when I pump the brakes, but then it will come on again.
Pedal is hard when I have engine running and at a standstill after pumping the brakes. Then I wait a few seconds, pedal goes to the floor. If I remove the check valve from the booster, I am hearing a hissing sound instantly, so it's building up some type of pressure, just not holding it.
Last edited by motc7777; July 12th, 2021 at 8:43 PM.
OK so when I start the engine, the pedal goes to the floor. Engine never dies or stutters.
Rectangular
The master cylinder I replaced it with is brand new. I'm bleeding the brakes from the master cylinder. Just a note: Tonight I was just using it around the property to move some things a short distance. when I was coming down the hill, covering the brake with my foot, it felt stiff like it should. But as soon as I stopped, went to the floor.
I just don't know if I really need a booster or not.
You don’t need a booster. If the pedal sinks when starting the engine, the booster works. You won’t gain anything by replacing it in terms of getting the pedal to stop sinking.
The older style reservoir matters because of the combination valve - having drum brakes I figured that’s what it was but wanted to ask for confirmation.
On valve body there’s a pintle valve that moves as pressure is applied to the hydraulic circuits - this valve is what actually meters fluid flow to the rear brakes, and to bleed them properly, it must remain depressed during the process. It’s not impossible to bleed the brakes with it fully functional but it takes much longer because the fluid flow to the rear brakes is so much more restricted.
The valve is mounted on the fender near the MC - follow the lines off the MC and it’s the first thing they connect to.
Your valve body may look a little different but it works the same. Typically the pintle valve faces forward or up, but there’s often 2 or 3 identical black caps, so to be sure you have the right one, have a helper press the pedal while you watch/feel the cap - if the cap is being pushed out with pedal movement, that’s the one you need to keep depressed.
There is a “special tool” for keeping it depressed (linked here -
Make sure to remove the black cap to make sure the valve stays fully depressed. Bleed them again and I’m sure you’ll get more air out of the rear circuits.
Last edited by Gumby22; July 12th, 2021 at 8:40 PM.
You don’t need a booster. If the pedal sinks when starting the engine, the booster works. You won’t gain anything by replacing it in terms of getting the pedal to stop sinking.
The older style reservoir matters because of the combination valve - having drum brakes I figured that’s what it was but wanted to ask for confirmation.
On valve body there’s a pintle valve that moves as pressure is applied to the hydraulic circuits - this valve is what actually meters fluid flow to the rear brakes, and to bleed them properly, it must remain depressed during the process. It’s not impossible to bleed the brakes with it fully functional but it takes much longer because the fluid flow to the rear brakes is so much more restricted.
The valve is mounted on the fender near the MC - follow the lines off the MC and it’s the first thing they connect to.
Your valve body may look a little different but it works the same. Typically the pintle valve faces forward or up, but there’s often 2 or 3 identical black caps, so to be sure you have the right one, have a helper press the pedal while you watch/feel the cap - if the cap is being pushed out with pedal movement, that’s the one you need to keep depressed.
Make sure to remove the black cap to make sure the valve stays fully depressed. Bleed them again and I’m sure you’ll get more air out of the rear circuits.
OK, these photos are at night, but I think I found what you may be talking about.
Which is on the other side of this.
I pushed it in while my son pushed the brakes in. I saw fluid come out, but it had a sort of white color to it along with the color of the fluid. Still, the brakes are going to the floor after multiple attempts.
Was I supposed to keep that held down while I bled from the master cylinder? Sorry I'm new to this. Also, I don't want to have to replace the Brake Booster if I do not have to, because it looks hard. I have not bled, at least not since I replaced the master cylinder, from all 4 wheels. Do I need to do that?
Last edited by motc7777; July 12th, 2021 at 9:50 PM.
OK, these photos are at night, but I think I found what you may be talking about.
Which is on the other side of this.
I pushed it in while my son pushed the brakes in. I saw fluid come out, but it had a sort of white color to it along with the color of the fluid. Still, the brakes are going to the floor after multiple attempts.
Was I supposed to keep that held down while I bled from the master cylinder? Sorry I'm new to this. Also, I don't want to have to replace the Brake Booster if I do not have to, because it looks hard. I have not bled, at least not since I replaced the master cylinder, from all 4 wheels. Do I need to do that?
1) Yes, that’s the pintle that needs to stay depressed while bleeding. Did it leak constantly when you pushed on it or it was initially a little bit of fluid and then nothing? If it’s a constant leak, I’d recommend replacing the combination valve unless you’re confident in being able to break it down and clean it properly. If not a constant leak, spray some penetrating oil on the valve and press the pedal a few times to work the valve in and out - that may stop the leak.
June 29th, 2021, 12:43 PM
1999 Silverado - Brakes going to floor.
