Poor brakes for the first 5 minutes

OzCharger69

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Hi all,

I've always had this problem but it's getting to me now.

69 Charger, front discs / rear drums, bendix brake double diaphragm booster, DOT 5 fluid recently flushed. Front calipers are big single pots from a Dart. Vacuum of 15 at idle

For the first few minutes after starting the engine, I really need to push on the pedal to get some sort of braking. Even idling on choke on drive it wants to push forward unless I'm standing on the pedal. It doesn't matter if the ambient temperature is freezing or warm. After a few minutes of driving and braking, brakes are fine. I can lock up the fronts with no huge effort. I'm just wondering if it has something to do with the brake pad material. Any input would be much appreciated.
 

1 Wild R/T

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It could be either.. When first starting the engine with your foot on the brake does the booster pull the pedal down as it should? After the brakes start acting normal try the same test... Is the results different? If so I'd lean toward the booster... If its the same I'd try another set of pads, Many HP compound pads need a little heat before they work properly...

Oh, when trying the test before starting the engine pump the brakes a few times to bleed of any vacuum reserve in the booster..
 

LowBikeMike

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If the problem gets better as the pads warm up I would lean toward pad composition. If the problem gets better after idling longer, not driving, I would tend to agree it is probably a booster issue. Does the pedal feel any different?
 

MoparLeo

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The old semi-metallic pads had this issue. Get a new set of organic or ceramic pads, resurface/swirl the rotors and then do a proper pad break-in.
The break -in is crucial on these. You are transferring friction material from the pads to the rotor surface. This aids in pads getting a "bite" on the rotors.
https://www.yourmechanic.com/article/how-to-break-in-brake-pads

The only real good thing about DOT 5 silicone fluid is it won't damage paint.
In DOT 3 fluid moisture is absorbed ,BUT it mixes with the fluid. dispersing it throughout the fluid.
DOT 5 separates. This means that it puddles in one spot. Not good.
Reconsider high quality DOT 3 or 4 fluid and just change it like you do your trans fluid and coolant. About every 3 years or 30,000 miles. Whichever comes first.
https://centricparts.com/getmedia/2...Centric-White-Paper-D5-Brake-Fluid-Basics.pdf
 
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Demonic

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I'll agree w Leo above. I had DOT 5 in my Demon years ago, made the pedal feel different. Didn't like it.
 

moparedtn

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What RPM is your cold high idle set at now? Sounds like a strong choke overcoming the brakes a bit?
 

OzCharger69

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Thanks for all your replies.

Vac source is where it should be, triple checked that.

Pedal behaving normally as in, the pedal pulls down when I start the engine. The behaviour doesn't change once brakes are warm.

Idle set at around 2000-2300 rpm

I had this problem when I was on DOT 4 fluid, hence the change to DOT 5


I'm leaning towards cheap pad material
 

MoparLeo

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I guess that I will answer the obvious question. Why 2000-2500 RPM idle ? Normal cars 700-900 RPM.
Must be more to this story you haven't told us.
 

OzCharger69

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My apologies I meant 1,200 rpm. What was I thinking?!?

rsz_pxl_20220106_001742244mp.jpg
 

Mopars & Missiles

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I could but that's not the issue at stake. At slow speeds on cold brakes I would rear end someone if I don't stand on the pedal.

Sorry but that most likely IS the issue at stake.

You are running the idle speed that high just to get 15" Hg. That 15" is a very minimum required for vacuum power brakes on a Mopar. Typically, you should be able to idle at 750-800 rpm and have around 18-20" Hg vacuum for proper factory brakes. To me that says you have too much cam for your factory power brakes. At that speed your carb is NOT operating on the fuel idle circuit. Almost certainly, you have the venturies partially open at that rpm.

I would guess that if you back the RPM down to a normal range of 750-800 your vacuum is most likely around 8-9" Hg? Way too low for vacuum power brakes. Too much cam IMHO.
 

Mopars & Missiles

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Another issue that plays into this is that it appears you have (or previous owner has) changed the brake system from 4 wheel drums to front disc/rear drums. (you state front discs from a Dart)

Factory drum brakes operate well with about 800 psi line pressure from the master cylinder. When you change the fronts from drum to disc, you now need around 1400-1600 psi line pressure on the front discs for them to operate well. The rear drums still need only 800 psi, hence the common usage of a "proportioning valve" in the rear brake line in order to equalize front to rear braking.

To try to compensate for the front discs need for higher line pressure, a SMALLER bore master cylinder is typically used. This results in somewhat higher line pressure, but also lower hydraulic volume flowing to the wheels. As a result, you will end up with a much lower pedal (longer pedal travel) to actuate your brakes. This approach is somewhat of a band-aid, sometimes it works OK, sometimes not so much.

So to summarize, it sounds like you have two issues with your brakes that add up to a lot of headaches. Good luck with your car, and yes the gauges are "cool".
 
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OzCharger69

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Thanks for the info M&M, my bad for not explaining myself well.

Cold start (on choke it) idles at 1,200 rpm. Once warm, idle drops to 650-700 rpm. Vac of 15mmHg is at 700rpm idle.

The car was originally a 4 pot front disc / rear drum car. The previous owner swapped the front calipers to an A body single pot calipers because he struggled to find pads for it and costs of pads (silly excuse if you ask me)
 

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