'69 Power Brakes

[NJ_Bob 10]

I replaced everything on my brake system & even had the power booster rebuilt and now I have a very stiff feeling in my brakes. The booster does seem to be working but the pedal seems to be too stiff & the brakes, while OK, don't feel like they hold that well. It's been so long since I had all new brakes that maybe I don't remember how it used to feel. (I'm the second owner)

 

Those of you with PDB, how do they feel to you? By the way, I have a big block..

[S code 69 13]

Lots of things it could be, are you sure you got the rod on the booster adjusted to the master cylinder correctly?

[69390 10]

I agree with S code, it could be alot of things. One simple thing to check is the vacuum line for the boost. Are you getting adequate vacuum in that line? Try this simple test, with engine turned off, put pressure on the brake pedal and then start the car. Does the pedal do down some when the engine fires up?

[NJ_Bob 10]

Lots of things it could be, are you sure you got the rod on the booster adjusted to the master cylinder correctly?

 

I'll have to look into this one, I'm not sure...

[NJ_Bob 10]

I agree with S code, it could be alot of things. One simple thing to check is the vacuum line for the boost. Are you getting adequate vacuum in that line? Try this simple test, with engine turned off, put pressure on the brake pedal and then start the car. Does the pedal do down some when the engine fires up?

 

What does it mean if the pedal does go down? Is that normal?

[NJ_Bob 10]

I wanted to bump this up again. Can someone give me an idea how their power disk brakes feel? Is your pedal hard, or spongy? Thanks..

[S code 69 13]

They should always feel firm, if they are spongy you probaby still have some air in the system.

[Denver69 10]

Sounds like I'm going through the same thing you are. We have a '69 that we added a SSBC power brake system to. The brakes were really, really good until about a month ago. I noticed a hissing and the brake pedal became progressively harder, almost like a block of wood was stuck under the pedal. And the car would not stop well. I called SSBC and the tech recommended I take a Mity-Vac and hook it up to the hose running from the intake manifold to the booster, pump the mity vac up to 23 hg and let it sit. It should hold vacuum for 20 minutes he said or the diaphram in the booster is bad. Mine didn't hold vacuum for 20 seconds on any of 10 tries. So I have a new booster on order and hopefully, I'll be back to excellent braking. If the Mity-Vac does hold vacuum for you, you might check how much vacuum is coming out of your intake. It sounds to me like you are not getting assist from the booster.

[NJ_Bob 10]

Sounds like I'm going through the same thing you are. We have a '69 that we added a SSBC power brake system to. The brakes were really, really good until about a month ago. I noticed a hissing and the brake pedal became progressively harder, almost like a block of wood was stuck under the pedal. And the car would not stop well. I called SSBC and the tech recommended I take a Mity-Vac and hook it up to the hose running from the intake manifold to the booster, pump the mity vac up to 23 hg and let it sit. It should hold vacuum for 20 minutes he said or the diaphram in the booster is bad. Mine didn't hold vacuum for 20 seconds on any of 10 tries. So I have a new booster on order and hopefully, I'll be back to excellent braking. If the Mity-Vac does hold vacuum for you, you might check how much vacuum is coming out of your intake. It sounds to me like you are not getting assist from the booster.

 

Hmm, I'll have to try that. But I first need to look up what a Mity-Vac is!

[NJ_Bob 10]

They should always feel firm, if they are spongy you probaby still have some air in the system.

 

It's the other way around, the pedal is hard & while it brakes on the highway, at fast idle I can hardly hold the car back around town.

[S code 69 13]

Sounds like a booster problem, check the rod length too that could be screwing up your travel.

[coz 108]

You know this could be more of a general vacuum issue. What is your vacuum at idle and say around 3k? Low vacuum at slow speeds will result in poor stopping. At highway speed you should generate enough vacuum to stop.

 

My cam causes me some low vacuum #'s at slow speed so I'm installing a hydroboost system to take the booster and vacuum out of the equation.

 

I also have PDB with upgraded SSBC front discs/rear drums and they stop well enough now so I agree with what the guys before have said. So check your vacuum off the mainfold if possible and also check the booster. I'm not sure what "stock" vacuum is suppose to be - mayber some can chime in but I have 10~12 at idle (I'd like 18) and around 22~25 under load at 60mph.

[NJ_Bob 10]

You know this could be more of a general vacuum issue. What is your vacuum at idle and say around 3k? Low vacuum at slow speeds will result in poor stopping. At highway speed you should generate enough vacuum to stop.

 

My cam causes me some low vacuum #'s at slow speed so I'm installing a hydroboost system to take the booster and vacuum out of the equation.

 

I also have PDB with upgraded SSBC front discs/rear drums and they stop well enough now so I agree with what the guys before have said. So check your vacuum off the mainfold if possible and also check the booster. I'm not sure what "stock" vacuum is suppose to be - mayber some can chime in but I have 10~12 at idle (I'd like 18) and around 22~25 under load at 60mph.

 

That's any interesting point, I've been playing with my timing to try & get rid of a hesitation that I have.. I have the ignition advanced pretty far (just barely pinging) & I forget, does that raise or lower the vacuum in that case?

 

I don't think that it's rod length, I had a pretty reputable shop send out the booster & reinstall it & the master cylinder..

[S code 69 13]

Normally more advance raises vacuum, and proper mixture will too so it's a good idea to set your idle mixture with a vac gauge.

 

My six pack Challenger only makes 10 inches of vacuum and the power brakes work perfect so unless your vacuum is way screwed up I doubt that's it. check it real quick and you'll know.

 

If you're sure the installation was done right I'd suspect a bad booster, in many cases where I've replaced something and it just isn't right I find a screwed up reman part as the problem.

[69390 10]

If the problem is mainly at idle, I'd bet it is a low vacuum issue. To test it, try advancing the idle screw to 1300 or so and see if brakes work better. If so, then you might have to get a vacuum boost to get good braking at low rpms.

 

Having said that, I still always suggest a good bleeding of the brakes before someone does everything else. For bleeding brakes, I highly recommend the mityvac. I have one and it makes it so easy! Also it is a US made product which is a nice bonus. If you buy a mityvac, be sure to get the new improved model with the swivel angled gauge and improved handle.

[S code 69 13]

Maybe I missed something, did you just rebuild the brakes and then your brakes were screwed up? or did they ever work right? if they worked before forget the engine not making enough vacuum.

 

Just check the vac and rule it out then proceed in finding the actual problem.

[NJ_Bob 10]

Maybe I missed something, did you just rebuild the brakes and then your brakes were screwed up? or did they ever work right? if they worked before forget the engine not making enough vacuum.

 

Just check the vac and rule it out then proceed in finding the actual problem.

 

My brakes haven't felt good in a while even after I rebuilt the drums & put new pads up front. I decided to let a resto shop rebuild the power booster & master cylinder for me. I've owned the car for years, but I can't remember if they ever felt good or not. I brought the car back & they said that the booster is working..

 

I did check the vacuum on Sunday & it's at 20". Also, I'm pretty sure that the shop used the original rod.

[S code 69 13]

Thats a ton of vacuum so forget that.

 

The rod has to be adjusted when you swap parts, the rod is part of the booster and mine was way out of adjustment when I bought my reman one. if it's seated too far out you'd get a situation like you're talking about, but there are other things as well. sometimes you have to take your best shot and do the process of elimination.

[LoneWolf2U 136]

Booster check, With eng off...pump brakes till all vac is discharged from booster.

Depress brake and hold, Start eng and run it at 2k rpm.

Did the petal dropdown slightly under your foot? Yes... booster is working.

 

The booster to master cyl rod MUST be adjusted to the master your using.

The rod being too short causes excessive petal travel and low braking pressure at wheels.

 

The rod being too long will cause brake binding after a few petal depressions,

The long rod will not allow the master cyl to release the fluid back into the master after petal released.

 

On my 70 Mach1, I can feel the brakes slowing the car down with just the weight of my foot resting on the brake petal without lighting the brake lights.

[NJ_Bob 10]

Booster check, With eng off...pump brakes till all vac is discharged from booster.

Depress brake and hold, Start eng and run it at 2k rpm.

Did the petal dropdown slightly under your foot? Yes... booster is working.

 

The booster to master cyl rod MUST be adjusted to the master your using.

The rod being too short causes excessive petal travel and low braking pressure at wheels.

 

The rod being too long will cause brake binding after a few petal depressions,

The long rod will not allow the master cyl to release the fluid back into the master after petal released.

 

On my 70 Mach1, I can feel the brakes slowing the car down with just the weight of my foot resting on the brake petal without lighting the brake lights.

 

OK. I'll try it on Sunday.. Thanks all.

[NJ_Bob 10]

I'm sorry that this is so drawn out, but what else is there to do on a Sunday night :-)

 

With the engine off, I pumped the pedal & then started the car. The pedal travelled down some. But them I noticed that the pedal height stayed the same (down a bit) & stiff, even after trying to pump it up & remainded there while I drive it. If I quickly turned off & restarted the car, the pedal travel still stayed down slightly and stiff. Only when I let the car sit for a little while did the pedal come up again.

 

Does anyone have any more ideas or suggestions for what to test next?

 

thanks!

[S code 69 13]

I'd pull the MC and check the rod length, if it's ok I'd pull the booster, unless you have so much air in the system that the pedal is going to the floor and your firm feel is bottoming out the pedal then one of those two places just about has to be your problem.

[dmac 10]

Yeah I agree with Denver69, it sounds like the booster is not functioning. That could be the pushrod from the pedal not being adjusted, or could be inside the booster causing no vacuum. I had a 76 F-250 that I went through the brakes on to try and fix the same prob, finally replaced the booster and searched for vacuum gremlins untill I got it to work right. S Code is right, the pedal should be firm, but not like a rock, it should give for a few inches with increasing pressure.