Hi folks-

I have a 74 Dodge Dart sedan with a 318 and power front disk brakes.
I noticed at some point that my gas mileage had dropped quite a bit, so I started doing the regular things like doing a tune up, checking the timing, etc. The carburetor I already know is in good shape. But my gas mileage stayed kind of low (I mean about 14 city 18 highway, when it had been more like 16-17 city and 20-21 highway). The change in gas mileage seemed to happen all at once.

So I began to wonder if my brakes were dragging at all.
I jacked the car up, and the front brakes do seem to have some drag- the question is, how much is normal?

I don't think the wheel would spin completely freely, i.e. if I spun the wheel I didn't think it would continue on for 10 revolutions or something.

But the right front in particular seemed to have more drag than the left front. i.e. as soon as I stopped trying to turn the wheel, the wheel would stop. The left front might spin a half revolution after I let go, then stop.

So I took the wheels off and looked at the brakes.
These are floating calipers.

I know some cars have sliding calipers, and you need to lubricate the slider pins, or the pads will wear out faster because they will continue to rub on the rotor after the brake pedal is released.

I pulled the calipers and pads off, and both front wheels do spin more freely than with the calipers and pads in place.

So my question is, 1. how much drag is acceptable, I am sure some is normal, and 2. what do I need to lubricate to make sure the calipers fully release?

I did spray the pistons in the caliper with silicone spray, and I put a light coating of silicone grease on the lands on the bracket where the pads sit.

Is this all I need to do?

I have noticed that the outer pads are slightly more worn down than the inner pads.

On sliding caliper cars, that means the sliding pins aren't lubed enough.
But what about on floating caliper cars like a Dart?

Thanks in advance for your thoughts.
-

Before looking at the wheels for drag, I would check to make sure that your vacc. advance is functioning correctly (if it leaks you are losing enough timing to make the mpg drop you are stating). I also would recommend a compression check to see how the rings are looking (you may have enough wear that a bore or two are 'low'). I would also use a vacc. gauge to tun the engine to the best vacc. reading (driving with a vacc. gauge in the cockpit will let you know in real time how the engine is performing (I can say it's interesting to lose compression on one cylinder and only see a 2-3" loss of vacc. on the gauge and have a 'choppy idle'...)

A late disc brake set up with new bearings and fresh grease usually I can get about 2 revolutions when spinning the wheel if adjusted correctly, if a bit tight maybe about 3/4-1 revolution. (When did you last to a bearing and seal change).

There is no "lube" for the caliper pistons (the brake fluid does this inside the bore). You can use grease or brake "quiet" on the piston faces to keep them from "singing/squealing". Brake wear can be attributed to a few different things mostly the rotor leaning to one side or the other (or making lots of turns one way...you're not Richard Petty, right?).

2 cents,

-D.idiot

Thanks for your ideas.
I did check the vacuum advance with a vacuum gauge, I am sure it is working.

But what I have not done has been to have a vacuum gauge with me inside the car while I am driving it around.

I could try that.

I did a compression check maybe 2 years ago, and the readings were fine- but that was indeed before the MPG drop. I could do that again to make sure nothing has changed.

I will say this. The quality of the idle has really dropped off quite a bit.
I haven't been able to narrow down why, though I did look for vacuum leaks in various hoses, etc.

However, the gears are turning in my head now... one thing I did not check for was to see if the vacuum booster has a leak in it which could be causing the rough idle, and, as you have suggested, a drop in overall engine vacuum which would in turn affect my vacuum advance readings.

Is there an easy way to test for that? Just disconnect it and plug the hole in the intake manifold, and see if the idle improves, and see if my mileage improves?

I haven't looked at the wheel bearings. That is definitely a possibility. I have only owned the car for two years, so the last bearing repack was done before I owned the car. That is something else I could check.

What I can say is that the pads themselves are worn normally, not at an angle or anything like that. It is just that I see that the outer pads are a bit more worn down than the inner ones.

I suppose it is just the retracting of the piston that is really the only factor here, right?

Thanks again for your help.

-----------------------------

>>Before looking at the wheels for drag, I would check to make sure that your vacc. advance is functioning correctly (if it leaks you are losing enough timing to make the mpg drop you are stating). I also would recommend a compression check to see how the rings are looking (you may have enough wear that a bore or two are 'low'). I would also use a vacc. gauge to tun the engine to the best vacc. reading (driving with a vacc. gauge in the cockpit will let you know in real time how the engine is performing (I can say it's interesting to lose compression on one cylinder and only see a 2-3" loss of vacc. on the gauge and have a 'choppy idle'...)

A late disc brake set up with new bearings and fresh grease usually I can get about 2 revolutions when spinning the wheel if adjusted correctly, if a bit tight maybe about 3/4-1 revolution. (When did you last to a bearing and seal change).

There is no "lube" for the caliper pistons (the brake fluid does this inside the bore). You can use grease or brake "quiet" on the piston faces to keep them from "singing/squealing". Brake wear can be attributed to a few different things mostly the rotor leaning to one side or the other (or making lots of turns one way...you're not Richard Petty, right?).

2 cents,

-D.idiot>>

Sorry, what I meant to say in the last post is that I am sure the vacuum advance is holding vacuum. I used a vacuum pump on it (not a gauge... though the pump has a gauge on it) and it held the vacuum.

However if overall engine vacuum is low then it is possible the vacuum advance does not advance the timing as far as it should.

Engine vacuum seems to be normal, at least at idle in Park.
But idle quality is not the best.

I think checking the brake booster to make sure that you don't have a vacuum leak there is a good idea, that would definately cause a 'leak'.



-D.Idiot

Thanks!