Need Help Troubleshooting Brake Troubles

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1973Duster340V8

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Hi All. I just replaced my Master Cylinder and Brake Booster in my 1973 Plymouth Duster. Everything seemed to go smoothly but I now when I tap on the brakes they compress to the floor immediately which causes the car to brake hard. I have to use my foot to lift the brake pedal back up so that I can continue to drive. Needless to say I'm not currently driving it anywhere... It's gotten me stumped.. I've been looking for info online about this happening to someone else but I'm not finding anything... Any ideas or suggestions?

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I would contact whoever you bought the booster from. Maybe they have some technical support. Most boosters have an adjustment rod that goes in MC. Could back that off a little and see if that helps
 
67cudafb said:
I would contact whoever you bought the booster from. Maybe they have some technical support. Most boosters have an adjustment rod that goes in MC. Could back that off a little and see if that helps
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Ok. Thanks so much. I will do that and see if it helps.
 
Russ Dyer said:
Sounds like the booster is not releasing.
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67cudafb said:
I would contact whoever you bought the booster from. Maybe they have some technical support. Most boosters have an adjustment rod that goes in MC. Could back that off a little and see if that helps
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Bad Sport said:
Sounds like the system may be full of air.
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I contacted Summit (who I bought the brake booster from) and they said that I might have a bad proportioning valve or I have air in the lines... We bled everything, so I'm almost certain there is no air in the lines.. Is there a way for me to make sure it is the valve before I order the part and go through the trouble of putting in a new one? Anyone have any tips on replacing one?
 
I would separate the master from the booster and see how it works. Studs are long enough to get a good gap there. I'm no expert but seems to me the booster would work right, pedal would return, even if there was no fluid or vacuum. I have seen a couple of bad valves. One that would lock up just the right rear wheel of a front wheel drive car with light pedal effort. The pedal always returned though.
 
I use one of these when bleeding the brakes. Best tool I ever bought except for my big mallet.....

Phoenix Systems 2104-B: V-5 DIY Reverse Brake & Clutch Bleeder | JEGS

Bled a lot of systems with this. Just leave the system dry, hook it all up, and it pushes fluid from the calipers and drums back to the master cylinder. No need to to bench bleed anything because it goes the opposite direction. Never had any issues with this thing and changed several MC alone, entire brake systems. Just pop the top off the MC and it does all the work. Never any air if done correctly.
 
TrailBeast said:
B.S. It's a bad booster.
Had this exact thing happen before.
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retractable said:
Call Booster Dewy He has great knowledge of power boosters.
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RedFish said:
I would separate the master from the booster and see how it works. Studs are long enough to get a good gap there. I'm no expert but seems to me the booster would work right, pedal would return, even if there was no fluid or vacuum. I have seen a couple of bad valves. One that would lock up just the right rear wheel of a front wheel drive car with light pedal effort. The pedal always returned though.
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So I ran a couple of tests after I got home from work... I disconnected the vacuum hose from the booster and plugged up the vacuum hose leading to the engine so I wouldn't have a loss of vacuum from the engine. When I drove the car without the booster it worked fine. I had to press the brake hard but the pedal did not compress to the floorboard. I then reattached the vacuum hose and the pedal compressed all the way down with the slightest touch, hard braked and wouldn't release unless I pulled it up with my foot... I also tried tapping the brake immediately after turning off the car to see if the booster would retain the vacuum and compress the pedal to the floor. It didn't. I don't think that this has anything to the with the valve and everything to do with the booster.... thoughts?
 
Booster retains vacuum after engine off only due to the little check valve where the line attaches. You should have had some vacuum retained in the booster if that check valve is good. Maybe just not enough to produce the extreme reaction to pedal movement?
My view... If master cylinder, proportion valve, etc.., works perfectly in a manual brake condition, vacuum assist has to be the fault.
 
Signs of a bad booster are hard pedal. The primary indicator that the brake booster is going bad or failing is a brake pedal that is extremely difficult to push. ... if it’s not returning then I don’t think it’s booster.

Check here:

Is the spring in the MC? Where the push rod is?

Proportioning valve: take one line off at a time and apply the brake. If no fluid comes out that’s where the issue is.

Also: bad check valve for booster.

Lastly, spray some lithium grease or wd-40 on the pedal where it rotates as well. Maybe just needs some lube?
 
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