Brake test

gsxrken · May 1, 2006

My power brake pedal is rock hard, but barely slows the car down at all.
After many searches, I figured bad booster, but with a mityvac it will hold 15 inches of vacuum indefinitely. I also have adjusted my booster pushrod out to the factory spec (.995").

The only thing that still has me suspecting the booster is that when I start the car with my foot on the brake (the classic test for bad booster, evidently), the pedal does not sink down at all. It's unchanged.

Any ideas? Can a booster still be bad even though it holds vacuum?

(BTW-Everything is bled (3x) and fluid comes out nice from every corner.)

rbohm · May 2, 2006

replace the check valve on the booster. that usually goes bad and causes the exact problem you have.

gsxrken · May 2, 2006

I would tend to think that the check valve is OK since that is what I am attaching my mityvac to and pulling vacuum (and I can blow through it one way and not the other)... but it is cheap, so I'll swap it out to verify. Thanks rbohm.

gsxrken · May 5, 2006

Well, it wasn't the check valve.
I have new booster on the way. I hope that does it.

2nd Mustang · May 5, 2006

gsxrken said:
Can a booster still be bad even though it holds vacuum?

Yes. I speak from personal experience.

gsxrken · May 13, 2006

Well it wasn't the gd booster, either. That is a lousy job, by the way.
I think the 10 inches of vacuum is the problem, but laying out 400-500 for hydroboost gets me mad just thinking about it.

MitchGT · May 13, 2006

they make vacuum canisters to help with this problem. I have not tried one myself, but it might work. Ask around maybe? you might also try adjusting the rod to a different length and see if it helps. I think factory specs in general are great places to start, but not necessarily the only possibility.

tweet66 · May 14, 2006

Sorry to hijack this thread, but what if you have the same problem with manual brakes, no booster?

gsxrken · May 15, 2006

In that case, I would suspect the wrong master cylinder bore size. If you have baked your drums real good on a mountain road or track day, you may have glazed the shoes- and this will also cause the car to have very poor braking action.

In my case, I am beginning to suspect my distribution valve... someone else suggested that the shuttle valve inside it may be blocking either the front or the back lines.

jerry S · May 16, 2006

How much vacuum are you pulling?

gsxrken · May 16, 2006

Jerry- 10 inches. I installed a comp cams vacuum cannister to store more volume, but at least in the driveway it made no difference at all.
http://www.powerandperformancenews....de=CC&Product_Code=5200&Category_Code=VAC_OIL

What did you do with your brake issues? Did you get that vacuum pump working?

jerry S · May 16, 2006

gsxrken said:
Jerry- 10 inches. I installed a comp cams vacuum cannister to store more volume, but at least in the driveway it made no difference at all.
http://www.powerandperformancenews....de=CC&Product_Code=5200&Category_Code=VAC_OIL

What did you do with your brake issues? Did you get that vacuum pump working?

My original set up was 4 wheel disks with a brake booster wired to an electric vacuum pump that then connected to the manifold. the vacuum pump has a switch that activates the pump when manifold vacuum is below 18 inches and shuts off the pump when the manifold pulls more than 20 inches. When my electric vacuum pump was hooked up, I had no brakes whatsoever. The pedal was hard as a rock and stopping was a nightmare. I found out that the pump was not working and I disconnected it and ran the vacuum line from the booster to the manifold. The difference was night and day. The only issue I have is when I slow down and manifold vacuum drops below 16 inches. I have a new electric pump and switch that will be installed next monday. So your problem is a vacuum problem.

Do you have a monster camshaft? Even if yes, you should be getting decent braking when you are at speed and only have problems when your engine slows down. If your pedal is always hard, then you have a vacuum leak somewhere.

69Rcode_Mach1 · May 16, 2006

Something isn't right. My car only pulls like 6-7 inches of vacuum, and the brakes are not great, but they still are noticeably better with the car turned on than with it off. Just takes some effort. I just bought a canister that will store some volume for me should improve it quite a bit.

gsxrken · May 16, 2006

Well, I hooked up a long length of vacuum hose from my Ford Ranger to my Mach 1's booster and guess what? The brake pedal behaves like it should. With my foot resting on the pedal, it drops an inch or two when the Ranger was started. And when I pulled the mustang back into the garage, I had one good power-assisted stop thanks to the stored vacuum.
So, since this custom camshaft is staying, I need to come up with either hydroboost setup or the elctric pump setup. Problem is, I've never seen anyone happy with the elctric pump, and the hydroboost is mega $$.

2nd Mustang · May 16, 2006

gsxrken said:
So, since this custom camshaft is staying,

...or get used to manual discs. My co-worker has a 69 Dodge Dart with manual discs/drums. He claims it is a little harder but not all that bad.

jerry S · May 17, 2006

gsxrken said:
Well, I hooked up a long length of vacuum hose from my Ford Ranger to my Mach 1's booster and guess what? The brake pedal behaves like it should. With my foot resting on the pedal, it drops an inch or two when the Ranger was started. And when I pulled the mustang back into the garage, I had one good power-assisted stop thanks to the stored vacuum.
So, since this custom camshaft is staying, I need to come up with either hydroboost setup or the elctric pump setup. Problem is, I've never seen anyone happy with the elctric pump, and the hydroboost is mega $$.

Hold your jets, Dash. Your engine should be pulling enough vacuum at 3500 rpm to run your brakes. Test your manifold vacuum at higher rpm and see if it pulls more vacuum than just 6-8 inches. If it pulls at least 16-18 inches (I have a pretty hefty cam myself and I still get 31 inches of vacuum at high rpm but only 10-12 at idle) then you will at least have power assisted brakes at 30 mph or more. If you pull only 6-8 inches when your engine is working hard, then you have a leak somewhere in your engine.

Let me do my electric vacuum pump install next week and report back on how the pump works. If my experience is positive, perhaps you can consider an electric vacuum pump.

At the end of the day, if I had to do it all over again, I would have gone with a JME dual MC manual brake set up.

rbohm · May 17, 2006

instead of an electric vacuum pump, how about an engine driven pump? you could use one off a 6.9 or 7.3 diesel pickup, and let it pull double duty, in that you can also use it to draw a vacuum in the crankcase. that will add back the power you lose by driving the pump in the first place, and perhaps more.

gsxrken · May 17, 2006

rbohm, I'm curious about the pumps you mention. Is it a fan-belt driven accessory, and have you ever seen one used in a musclecar application?

jerry S · May 18, 2006

gsxrken said:
rbohm, I'm curious about the pumps you mention. Is it a fan-belt driven accessory, and have you ever seen one used in a musclecar application?

Unless you can find something from a junkyard from one of the diesel pickups rbohm mentioned, the moroso units are rather spendy. they start at $360 and go up to $550. Summit p/n MOR-22640 and MOR-22642

jcode68 · Oct 6, 2006

gsxrken, I'm wondering how this story ended? I have a similar problem (same symptoms you describe) except I'm pulling 18 inches of vacuum at idle, so I'm not sure where the problem lies.

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