My Power Disk Brakes are frustrating me...

-
JoesEdge

JoesEdge

Well-Known Member
Joined
Jun 6, 2013
Messages
1,110
Reaction score
285
Location
Downey, CA
Ok, so I have a '74 Duster with power brakes, disks in the front and 10" drums in the back. The engine seems to only want to give me a max of 10"/hg of vacuum.

Anyway, so when I push the brakes, the the first push is good but the subsequent pushes I need to pump it. Regardless, I have to pretty much push the pedal to the floor to get the car to stop. Locking the brakes only seems like a fantasy at this point.

This is what I've done so far. I've replaced the rear wheel cylinders, I have flushed and bled the brake lines.

I could probably adjust the rear drums a bit tighter since I only adjusted it enough so that the drums can snugly slide on. Although, I don't think that will fix my problem.

At this point I don't know if my problem is any of the following:

  1. Not enough vacuum to operate the power booster.
  2. The booster is possibly bad
  3. Calipers are bad

If the booster is good and the calipers are good, then should I switch to manual brakes or swap the cam? I do want a better idle actually and I do kinda like power brakes. Kinda got used to them in my modern rides.

I could post videos and pictures if needed.
 
10"/hg of vacuum is way to low to operate the booster properly, I think 14 is the lowest and you can still get a hard pedal after 3 or so pumps

factory engines run 16+ "/Hg, you have a few options, add a vacuum pump to allow your brakes to work properly, change the cam; or remove the power booster and put a manual master cylinder on it
 
You don't need power brakes on these cars. Ever since I deleted that big can, I'm happier with the brakes; better travel, better pedal feel, better control, even with a power master cylinder. Don't miss it at all. No strong legs required. F*** it', just another thing to get in the way.

Round file that booster, don't look back.
 
you need to find out why you only have 10" of vaccum.do you have a wild cam,vaccum leak,you should have a lot more for a stock 74 duster.assumming its stock,check your hose going to the booster,could be cracked
 
jdsduster said:
you need to find out why you only have 10" of vaccum.do you have a wild cam,vaccum leak,you should have a lot more for a stock 74 duster.assumming its stock,check your hose going to the booster,could be cracked
Click to expand...

he meations a different cam in his first post; I'm assuming its something quite radical with no idle quality's, if not he has a very bad engine vaccum leak or something
 
The pedal shouldn't go to the floor with any brake system.
A brake booster does nothing more than ease pedal effort. Complete lack of vacuum ( like engine off ) should make the pedal brick hard in effort but the pedal would still be at the top. To tell us what the pedal does with engine off might help.
 
jdsduster said:
you need to find out why you only have 10" of vaccum.do you have a wild cam,vaccum leak,you should have a lot more for a stock 74 duster.assumming its stock,check your hose going to the booster,could be cracked
Click to expand...

Bought the car in December "as is" which a Holley 750 DP on a single plane manifold, which I switched to an Edelbrock 750 cfm that I found on Craigslist because I couldn't get the thing to idle right. That helped TONS!!!! The cam seems pretty aggressive since the idle is very choppy. I don't know the specs of the cam.

66plyValiant said:
he meations a different cam in his first post; I'm assuming its something quite radical with no idle quality's, if not he has a very bad engine vaccum leak or something
Click to expand...

I did mention changing the cam, but I think I should have mentioned the current cam seems pretty aggressive. Damn thing idles happily at about 1100 RPM, and even that's choppy.

As far as vacuum leaks go, I pulled 5-10"/hg with the Holley and about the same with the Edelbrock. The carb sits on a single plane MOPAR manifold. I don't see where there could be a vacuum leak unless it's leaking by the intake-to-head gasket. Is that possible? I could re-tighten the manifold. The carb is on pretty snug/tight.

I'm going to try the other suggestions. I need to run to the hardware store, when I get back I'm going to do the following:

  1. Tighten up the rear drums.
  2. Check the feel of the brakes with the engine off

I'm pretty sure that with the engine off, the pedal is stiffer with a couple of pumps. I'll have to double check.

I know I can swap to manual, but choosing the right master cylinder is quite confusing. I mean, I don't know what size to choose. I like those aluminum master cylinders and I always see them at the junk yards. I know I'd need an adapter, but I also read that I need the "manual" brake rod. Where could I find that anyway?

Anyway, I'll post back my findings and a few pics ASAP. I really want to get this thing streetable. Not being able to stop on a dime as sort of been my...uh...show stopper. lol
 
I agree with the masses 10" is to low how about an electric vacuum pump like the on SSBC sells it would be cheaper and easier than a cam swap and if you like the way the car runs and it's just a matter of the hard brake pedal it may be the way to go
 
hemi446 said:
I agree with the masses 10" is to low how about an electric vacuum pump like the on SSBC sells it would be cheaper and easier than a cam swap and if you like the way the car runs and it's just a matter of the hard brake pedal it may be the way to go
Click to expand...

I kinda don't like how it runs. The whole car shakes. Everything rattles. It's kinda annoying. Half the time I want to pull the engine out, and blow it apart to see what I have inside. There's some knocking going on too. Not sure if it's piston slap or what.
 
Ok, so with the engine off I pushed the pedal and it soft at first until it gets firm. I let it roll backwards in the driveway just to see where the brakes catch. Seems like it catches near where it gets firm. How would I know if the master cylinder or booster is bad? How I can I check?
 
You should have about an inch of free travel at the top of the pedal stroke.
After that it should make brakes. Hold the pedal there with pressure and wait. If the master cylinder is bad the pedal will slowly fall.
 
Ok. it's getting too dark to work on it. Then I got side track working on the Z-bar. The whole clutch linkage was rattling and I noticed that everything is loose. I tried removing z-bar, but it's a bugger to get out. That's a story for another thread.
 
You can ditch the booster and just use the same master cylinder thats on the booster now. But you will need a manual disc brae pushrod as the pushrod set up on the power booster is way different. Either one from a non power booster a body or an adjustable push rod from Mancini.
 
Try throwing in a set of rhoades lifters. They will increase vacuum, smoother idle at lower rpm, better mpg, all without sacrificing your top end. The part Number for mopar v-8 is 2018



I used them in a 340 wit a MP 484/284 cam and it ran much better. Before Rhoades lifters it was choppy idle at 1100 RPM, with Rhoades lifters, it would idle smoother at 800 with more vacuum.


Rhoads Original hydraulic flat tappet variable duration lifters are famous for increasing low-end torque, engine vacuum, and idle quality on engines equipped with performance hydraulic cams, while maintaining maximum top-end power. Better fuel economy and improved emissions quality are often reported by customers with Rhoads lifters. The unique construction of Rhoads Original hydraulic lifters reduces lift and duration at idle by approximately .010 in. to .020 in. Duration is reduced by approximately 10 to 15 degrees. Total lift and duration are restored at approximately 3,500 rpm. Original Rhoads lifters are also anti-pump-up for higher rpm revs. Typical vacuum increases range between 1 and 3 in. Rhoads lifters can be used with either adjustable or non-adjustable valvetrains. Original Rhoads lifters, with their legendary "ticking" at idle, sound similar to solid lifters. These Rhoads Original series lifters are designed for use with high performance street, marine, or racing applications.


Go to this page and read the second article "the smart lifter" to see how they work:

http://www.rhoadslifters.com/Pages/Articles.html


You can get them here:

http://www.summitracing.com/search?...rtOrder=Ascending&keyword=rhoades lifter 2018

You can try using them with your current cam or another one, but they will help you idle smoother and get more vacuum. I've been using them for 25 years with no problems.


*********************************************************

I also agree as stated above to first adjust the drum brakes, then bleed them to get the firm feel in the pedal.
 
66plyValiant said:
he meations a different cam in his first post; I'm assuming its something quite radical with no idle quality's, if not he has a very bad engine vaccum leak or something
Click to expand...



sorry i must be blind,all i seen was should i swap to a differant cam.i quess i should have assumed he already swapped cams.
 
maybe the engine is not healthy, bad cylinders, bent valves, he does here a engine noise?
 
The engine sounds healthy, but I think it's the cam that came with the car when I bought it. It's pretty choppy. I Check the manifold bolts and all are pretty tight. Shouldn't be any vacuum leaks around the carb either because I replaced that. I'll try to do some more work on it this weekend. Probably Sunday.
 
67Dart273 said:
WHERE is the timing?
Click to expand...

That's another problem. I have marked TDC half a dozen times and verified that the rotor is pointing at the #1 cylinder, but every time I put my timing light on it it shines NO WHERE near where I marked it. It's been frustrating. Keep in mind, this engine has an MSD distributor and ignition. Maybe it's my timing light? It's NOT a dial back timing light. I was thinking of trying another to see if it behaves differently.
 
JoesEdge said:
That's another problem. I have marked TDC half a dozen times and verified that the rotor is pointing at the #1 cylinder, but every time I put my timing light on it it shines NO WHERE near where I marked it. It's been frustrating. Keep in mind, this engine has an MSD distributor and ignition. Maybe it's my timing light? It's NOT a dial back timing light. I was thinking of trying another to see if it behaves differently.
Click to expand...

i have a 50 year old craftsman timing light, and it works with MSD boxes just fine...you're not trying to set it with a vac advance hooked up are you?

you need around 10-12 at idle, and then it should advance to around 32-36 fully advanced. thats with the vac advance unhooked, and all ports plugged on the carb. If your distributor only has a mechanical advance, then get it to idle, and see what it goes to when advanced. on street cars i go with the numbers above, on my race car, i lock it out at 36 with no advance and leave it. but those are two very different animals.

you have some odd things going on in your combination...the single plane intake, and the "high lift" mysery cam aren't doing you any favors on a driver.
 
johnnymac2k6 said:
i have a 50 year old craftsman timing light, and it works with MSD boxes just fine...you're not trying to set it with a vac advance hooked up are you?

you need around 10-12 at idle, and then it should advance to around 32-36 fully advanced. thats with the vac advance unhooked, and all ports plugged on the carb. If your distributor only has a mechanical advance, then get it to idle, and see what it goes to when advanced. on street cars i go with the numbers above, on my race car, i lock it out at 36 with no advance and leave it. but those are two very different animals.

you have some odd things going on in your combination...the single plane intake, and the "high lift" mysery cam aren't doing you any favors on a driver.
Click to expand...

Let me borrow your light! LOL Yeah, I thought about buying another but I wanted to check the return policy before I went that route.

I have an Edelbrock Air-Gap in waiting to be installed. I just need to order the gaskets and bypass hose inlet. The dual plane should help. Not sure if it will help with vacuum, but hopefully help the drivability.

BRAKE UPDATE

Since many have suggested to ditch the Power Booster, I'm going to do just that. Seems like it will make life easier.

Today, I called Doctor Diff and ordered his Aluminum Master Cylinder kit with the adapter. I also ordered an adjustable manual push rod and master cylinder brake lines.

I ordered the 1 1/32" bore master cylinder because supposedly that's what "standard" for disc brakes according to Doctor Diff. I was afraid that the 15/16" would have too much travel. I hope I made the right choice. It's tough to make when you really don't know the difference.

I actually met a guy in Dana Point, CA (bought a 4 speed transmission from him, more on that some other thread) over the weekend and he had the Doctor Diff setup on his Duster, which was a very nice '74 Duster. I wish I took a picture.

I plan to put it all in this weekend. I'll document as I go.
 
with the timing you may need an adjustable light to be able to see where your sitting at idle, could be that its flashing around 16+ which is hard for a normal light to pick up on
 
66plyValiant said:
with the timing you may need an adjustable light to be able to see where your sitting at idle, could be that its flashing around 16+ which is hard for a normal light to pick up on
Click to expand...

You could also hold it at 3000 RPM, and set the timing at 32 degrees there. you should be well fully advanced by that point. tighten it down there, then check it at idle. as long as its running ok, and isn't idling more than 14 or 15...i'd leave it.
 
You must log in or register to reply here.
-

Back
Top