• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Power Brakes not working

Data Plate

Well-Known Member
Local time
2:00 PM
Mar 23, 2021
Reaction score
New York
Im trying to install a power brake booster and master cylinder on my 69 Charger RT 440, having a hard time getting the power assist to work. I bench bleed the master and installed the power booster then bleed the brakes at the wheels and still no power brakes, I had the rod adjustment all the way in- then half way then- all the way out and still the same results, I even got another power booster because I thought maybe the first one was bad, What am I doing wrong
Good question. The simplest way to see if your booster holds vacuum is to rest your foot on the brake while starting the car. As soon as the engine starts the pedal will fall a little bit.

Why do think the power isn’t working? Still need more brake effort than what you think? Power drum isn’t like a modern car. Do you have disks?
I did try that and it doesn't drop. I pulled the vacuum line off and it seems to be pulling good vacuum. I do not have disc. not sure what's wrong
Make sure the check valve [ booster body, where hose attaches ] is working.
Is this an OE Booster or one of those aftermarket units?
Just a pure dee guess, but you still
have air in your brake system. Also,
check your master cylinder
application to make sure it is correct
for drum/drum. Most parts guys
assume you have front disc/rear drum.
How do I check the check valve I pulled it off and it seemed to have good suction . Is there another way to check it I guess I could put a vac gauge on it what should it be like 15. Booster is aftermarket I bought it from Roseville motors in MI. I will check the master with the company I didn't think of that maybe they sent me the wrong one. Thanks everyone
also the new one says 1.25 bore size is that correct for me
1.25 is large. factory drum was 1". i've always had big bore issues and avoid them. big bore and low vacuum from poor tune-up or bigger camshaft equals no brakes.
so if drum was 1" and I have the 1.25 what should I do now the car does have a mild cam but nothing crazy
so if drum was 1" and I have the 1.25 what should I do now the car does have a mild cam but nothing crazy
i use a 1" with power brakes and drums. what's a "mild" cam? i've had wheel cylinders and the distribution block clogged up from rust using dot 3 fluid. that'll definitely hurt the brake system.
I didn't put the cam in but the guy said like a 450 lift
You can't just trickle out tiny bits of info. You need to make it easy for the members to help you.
I have had far more than my share of problems with brake systems but mine have revolved around non stock systems.
A basic front disc, rear drum system is not too complicated. You will figure this out.
Manual brakes have fewer moving parts to worry about but often times, you can't get enough pressure to the brakes without some type of assist.
The vacuum booster is the way that Ma Mopar equipped our cars to address the need for assist. Everything from the slant six to the Even the famous HEMI had a mild enough camshaft to run power brakes.
The 440 Magnum had a .450 lift cam and was more than capable of producing enough vacuum to power the booster.
One description that I have used to describe poor braking is this....The force applied to the pedal does not translate to adequate force at the wheels.
To be clearer, you force the brake pedal as hard as you can push but the brakes simply do not adequately stop the car.
I have lived it. I have tried many things but my situation was with a 4 wheel disc system.
For you, it could be as simple as a bad or improperly adjusted booster.
The check valve is only supposed to allow flow toward the engine. This means that suction from the engine draws air from the booster to the engine, thus powering the booster. If there are leaks in the housing or the internal components, you will not have enough "boost".
Someone above mentioned how the brake pedal should drop once you start the engine. This is correct and one good way to determine if your booster is functioning. I recently tried a hydroboost setup in my car and while the manner of boost comes from a different source, the principle is the same. My pedal never dropped and the hydroboost never provided any assist.
As weird as it may seem, it is possible to get 2 bad boosters in a row.
To address a couple of your questions...
No, a proportioning valve isn't the cause.
I suspect that you may have read your specifications wrong. I have never seen a 1.25" master cylinder. You may have read 1.125. 1 1/8" is still pretty large. I run a 15/16" unit in all of my cars. Larger bore master cylinders make for a real hard pedal.
Yes your right it says 1.125 so I should get the 15/16 master cylinder or try a new power booster this is my second one in there.
M/C bore size is definitely hurting you....
But a quick test.. Engine off, push the brake pedal 4-5 times, leave your foot on the brake pedal after the last stroke with light to moderate pressure applied.... Now start the engine.... Does the pedal move? It should sink a little then stop... If it does the booster is doing something.... It may or may not be doing enough but at least it's doing something...

Since your booster is aftermarket if it's this style


I wouldn't expect much.... I've worked on a few cars with those, the first car I put three boosters on & finally bolted on a used original booster that fixed the problem.. Since then I go straight to an OE booster...

Talked to a buddy that owns a local performance shop, his guys had swapped dozens of those under warranty & finally started converting cars to Hydraboost.. Till they got a bunch of bad Hydraboost units & then they were really screwed.... Wound up eating a lot of repairs to keep from getting sued.... There is a reason a lot of shops don't want to get involved with aftermarket parts Hel the OE stuff these days create enough come backs....
When I pump the pedal and hold it and start it there's no drop in the pedal. This is what I bought from Roseville Moparts

When I pump the pedal and hold it and start it there's no drop in the pedal. This is what I bought from Roseville Moparts

View attachment 1420938
I have zero experience with the new Chinese knock offs of the original Bendix Booster.. I know the real ones work good... I don't know if the new Chinese ones act like most Chinese stuff of if they got it right... The fact the pedal doesn't move isn't very promising....
If the booster is leaking vacuum your
engine may be running slightly
rough and rich. Try disconnecting
the hose from the booster and
blocking the hose off with your thumb.
Your engine idle should smooth out,
which means the booster is bad.
If it doesn't, there's either a crack in
the hose or a bad connection at the
vacuum source.
This of course,
assumes all other vacuum lines
and/or connections are good.
Another leak source to check is
the intake manifold.
Last edited:
Ok thanks ill try that who's sells original boosters can you still get them
Auto Transport Service