1975 setup (in my 71) with 302 edelbrock carb plumbed to my brake power booster, front disks with rear drums. I recently changed my rear rubber hose to a longer one so I can go to dual exhaust. I've done many brake jobs in the past and understand the bleeding procedure. With the car off, I press the brake pedal and it travels down normally and holds firm at a specific location. However, this specific location is actually a little further inward from where it was before. As I'm firmly holding the brake pedal, I turn on the car (leaving it in park) and the pedal travels down further and almost all the way down to the floor board and feels it's bottoming out. With the pedal to the floor board, it is quite noticeable the engine idle bogs down almost chocking the engine to turn off. When I release the brake pedal, the engine returns to normal idle. With the engine still on, I press the brake pedal (pumping it) and it continues to travel to the floorboard and chocking the idle. By driving it, I learned that the car brakes but not enough stopping power as it did previously. So, it's not safe to drive right now. No leaks are present. I checked the fluid level in the master and it's in it's normal level. I rebled the rear brakes a couple of more times, still the same. I checked for vacuum leak, new hose. Since I'm not sure how to tell if my booster or master has gone bad, I can't confirm if that is bad. It seems strange before I changed the rear brake hose, this was not an issue. I did not bleed the front disks because master separates front to rear. Any responses that leads to a procedure to test to confirm would be helpful and appreciated. Thanks.
I have had problems before with the brake booster. When stopping the diaphragm looses so much vaccum that the engine would idle slow or the engine would die. I don't know if this his problem or not.
With a bad booster (from my experience) The peadal will be hard and not travel far or it will be super touchy. I think you have a bad master cylender.
Neither...The booster gets its vacuum from the engine. When the brakes are applied, the vacuum bleeds off. This can pull the idle down. Especially if its idling too low to start with. Also the check valve could be bad causing the vacuum to bleed off prematurely...This will pull the Idle down due to increased demand from the booster. The check valve is attached to the booster and the vacuum hose from the engine is connected to it...Soo the master wont do it but the booster can.
If the vaccum booster diaphragm is blown the engine will idle normally. When you mash the pedal it will cause a severe vaccum leak. From my experience as a mechanic for about 35 years.
Dude...in your last post...You asked if the master cyl would pull down the idle...I'm 46...been turning wrenches (on cars) since I was 10. thats 36 yrs...(Big deal) I refuse to be 10 again. The master cyl...Will not pull down the Idle. Can we play nice now???
Mavman72. I am not going to argue with you. The guy asked a question and I said I am quessing the brake booster. I did not say it was the booster only I quessed it to be. I asked if the master cylinder would pull the engine down. You are entitled to your opinion as well as I am. I was only trying to help. It appears on this board if some one tries to answer a question for someone then some one want's to argue about it. I Quess this will be the last time I will try to help. Thank You
All, thanks for your replies. After reading several posts, I failed to mention. With the engine running at idle, I press the brake pedal and I hear a slight vacuum leak. This could be the issue and will check on it tonight. So, if I do need to buy a new booster check valve, do they sell that separately? If they do not, what would you guys recommend? I'm gonna check the rubber grommet also. Thanks guys.
Mr Hopkins...Please dont stop helping on my account. I apologise if I ruffled your feathers...I was merely defending my previous response...Nothing personal man. This place needs all the sharp minds that are here...Its what makes this forum so great. Pete
You proved the brake booster is good. You test it by pressing hard on the brake pedal. Then start the engine. If the pedal goes down the booster is good.
