zodiacblueS

How much vacuum is required for the brake booster?? My '73 has an original booster rebuilt by Steve at Brakeboosters, but my pedal is hard as a rock. The 455 has an Edelbrock Performer intake, and an Erson TQ40 cam, which is fairly mild. I can get a gauge to check vacuum, but would also like to check the booster to see if it holds. How can I check the booster? The system has been bled, new MC bench bled, and I am a little frustrated. The pedal was working fine for the bleeding process, with the engine off. And I think I got all the air out. I can get some brakes, but really need to push hard. Thanks in advance.
The booster vac line is the metal one switching to rubber past the manifold, but you already knew that.

Allan R

It's not likely the booster or the master cylinder. More likely than not your power piston on the disc brakes is seized. Pull the calipers and rebuild them. Rebuild kit is about 8bucks/side. I had exactly the same problem with my car and that's what it was. BTW, that's also a good time to change out your brake hoses, and pads if they're worn. While you're at it, pull the front rotor and repack the wheel bearings. Sounds like there's a lot to do, but it's really not. Work at your own pace, but unless you're restoring everything to new condition this won't take long.

zodiacblueS

Thanks Allan. Napa has the complete rebuilt OE calipers for about $20 a side, so I went ahead and ordered a pair, and will pick them up tomorrow. My bolts are corroded, so I think its best to go ahead and replace the units, and cheap insurance. As I think about it, I should be able to push through the booster even w/o vacuum. Right? Now to gease the bearings.

Allan R

IIRC NAPA calipers come with new mounting bolts and banjo bolt crush washers? So double check when you pick them up. Should have a parts bag inside the box (or bag) it comes in. 20 is a helluva good price for calipers; that's after core right? And you don't have to worry about the bleeder seizing either. I think you'll feel the difference with those new ones right away.

zodiacblueS

Right. Cores are $17.50 ea. Like I said, cheap insurance. Will advise. Thanks again Allan.

Rocketbrian

I'll be very interested to hear about your results. My 71 SX has always had a hard pedal and if you slam them on they feel slow to react and won't ever lock up. My calipers are original. Hope replacement fixes your problem.

Allan R

Brian, I've driven my car since it was 2 years old and remember what a difference it was going from power drum to power disc. My car developed the hard pedal because of non use. It was easy to see why when I rebuilt the calipers. When I reinstalled and bled them? Night and day. My calipers were original too. Be careful of the bleeder screws, they could easily be corroded shut. They can be a $%*# to get out and replace.

zodiacblueS

I spent a few hours changing the original calipers out to rebuilt units from NAPA, and the hard pedal is still there. No difference can be felt. I don't mind the time, or the $40. 40 year old calipers probably should be replaced. And my bearings are now cleaned and full of new grease. Could air in the line be causing a hard pedal? I always thought air gave a soft or spongy pedal. Anyway, are there any other ideas out there. Thanks in advance.
Lem C.

m371961

I had the vacuum line going to a booster colapse on the inside once. Could not tell from looking at the outside of the line.

Allan R

Lem,
I really don't know what to say. I was sure it was the caliper piston. You said you already had the booster rebuilt. Your MC is not leaking? Full MC reservoirs? I'll wait and see what others might chime in with.

MDchanic

This is a tough one.

I think it's time for basic troubleshooting.

Is there vacuum at the end of the vacuum line where it connects to the booster?
You should be able to disconnect the line, hold your thumb over it, start the car, slowly remove your thumb, and feel a real rush of air before it's open enough to stall the motor.

- Eric

Allan R

I see where you're going with this Eric. I didn't question the BB vacuum because he said it had been rebuilt. But if the diaphragm has failed....

Lem, When you unplug the big line from the booster it will almost kill the engine if it loses that much vacuum so fast. Just saying so you're prepared to block it off with your thumb really quick. (Learned that when I tried Seafoam on my 98)

Eric's suggestion I think you need 2 people to do that unless you have incredibly long arms.

brown7373

It spounds to me like either the booster leaks or it isn't getting vacuum. But if it was leaking so bad that the booster didn't work, the engine would idle very bad or not at all. I rebuilt an engine on a Pontiac once, and when I got ready to drive it, I realized the booster wasn't connected to the vacuum, and I didn't have a piece of hose. So I decided to drive it to the parts store to get a hose. I almost broke the seat back pushing on the "now non assisted" power disk brakes. They don't require a lot of pressure, they require Superman to stop the car. Are yours that bad?

zodiacblueS

Eric, I just checked the hose to the booster and while I don't have a vacuum gauge (I'll pick one up tomorrow) there is significant vacuum on the line, so that my thumb feels the pressure when I take it off the hose. I had my Booster redone by Brakeboosters.com, so I think the booster itself is probably good. What next? Thanks. maybe the proportioning valve went bad? The brakes were fine before I did my rebuild.
Brown, I relate to the force required. I'm a big guy, but I still wouldn't take it out as is.

zodiacblueS

When I take my thumb off, the idle increases slightly. Engine doesn't seem to mind the leak? I just checked my timing and I am about 10-12 BTDC

MDchanic

I'm betting on a bad booster or something very odd about the booster installation.

- Eric

zodiacblueS

Does anyone know how to check the booster. I can call Steve (Brakeboosters) tomorrow, but I have to assume he checks them before he sends them out.?
Thanks for your time.

Allan R

Zodiac, you don't need a vacuum gage for this. You already determined that you have ample vacuum in the line. Highly unlikely you did anything to the booster or MC during your rebuild. Did you by any chance put the clevis pin in the wrong hole on the brake pedal when you reattached the BB pushrod? If I remember right there are 2 holes.

Did you redo your rear brakes too? Is the drag adjuster set properly?

Like you, I'm a big guy and I could definitely feel something wrong when my brakes went. I also ended up replacing my Master Cylinder because it blew out on me. Possibility??

zodiacblueS

Allan, I looked at the holes in my pedal and I think you nailed it. I have three holes and I think I should be in the top hole, not the bottom. There appears to be some wear on the upper hole. (picture attached) Now that I think about it, it did go on a lot easier than it came off. I even had to loosen the booster to get it off. Looks like the geometry will be much better. I'll check the assembly manual in the morning and let you know. I love it when it's a simple stupid mistake.
Thanks a lot to all. If I had one, I'd offer you a Red Cap.

Allan R

I'll wait till tomorrow when you reposition it and report back.

MDchanic

Man, that sure looks like it.

It's always something simple...

- Eric

zodiacblueS

Thanks Eric!

Nasty455

Just a side note...I replaced a master cylinder,not long ago,and although I bench bled the unit,once installed,the pedal was rock hard like you say.
I ordered a new booster,then re bled the master cylinder on the workbench,and re installed all the parts...brakes work great now.
Could be either,neither or both...check your assembly.

zodiacblueS

I spoke with Steve at Brakeboosters.com and he gave me two tests for the booster function. 1) started the car and ran for a minute, then shut it off. I waited a couple of minutes, then pulled then check valve from the booster and heard a significant whooshing of air going in, indicating the booster was holding vacuum. 2) I disconnected the MC from the booster, started the car and pushed the brake pedal. The booster pushrod went out quite smoothly, without effort. A second indicator that the booster is good.
The pin on the brakepedal that the pushrod slips over is pressed into the current hole and doesn't come out, so that's where it was to start. I'll bleed the system again, but without much hope. Any other ideas? It sat for about a year while I did the rebuild. I'll run down all the brakelines and see if there's something obvious.

Allan R

Well, about the only thing you haven't done with this thing is replace or rebuild the Master cylinder.

Your car has disc brakes right? Can you check and see how much of the pads are left?

zodiacblueS

Factory front discs, pads are virtually new. The MC is new (rebuild) from NAPA and was bench bled. I'm going to pull the lines from the MC to the combination valve one at a time and see if I get good flow when pedal is pushed. Then move downstream from there. I'm thinking something is blocked somewhere, or pinched. It's just a bunch of tubes at this point, but it will be a hassle. Brake fluid is fairly cheap, and will soon be all over my garage floor. Will keep you guys posted. Thanks, Lem

Allan R

Be careful with that brake fluid, it's corrosive as heck on paint.

goatwgn

I have seen sticking proportioning valves cause a problem like this as well.

Allan R

Zodiac, anything new to report on this?

zodiacblueS

Well, I rebled all the brakes with the engine running and my BW pushing the pedal. Got good flow at all the wheels. Then I replaced the rubber vac line from the check valve on the booster to the steel tube connected to the manifold. The original was really stiff and I don't see how it could have sealed well. If I run the engine a bit the vac seems to "build up" adequately to get good pedal action. I tested the line and have about 15 to 16 inches vac at the booster hose at idle. I have read most boosters prefer 17 inches and up to work well. My cam won't allow more the 15 or 16. I have been toying with the idea of a vacuum reservoir, fairly cheap, ~$40, and small 5"dia.x 7" long, that will hold developed vacuum for for use by the booster. I'm fairly certain the booster is good, just not enough vac generated in my rebuild. AARRRGGGHHHH!!!
And Goat, the only piece I haven't replaced is the proportioning valve. Are there moving parts in there?
I read on the 73-77 forum that boosters like 20" vacuum. If that's correct I definitely need some assist.

Allan R

Zodiac, something you said earlier is bugging me. It was If you've replaced the major components of your brakes with new ones and you still have a hard pedal, there's something that's been missed somewhere. Bench bleeding the MC isn't it. If anything with air in the MC you would have a spongy pedal. The BB is new and holds vacuum. Your proportioning valve should be ok, but is the only thing you haven't checked?

What about the rear brakes? Did you do any work to them, or was it only the fronts? If you did the rear brakes also, I'm thinking there has to be something with the wheel cylinders or spring hookups back there that was done 'wrong'. Rear brake problems can also affect the brake pedal effort.

I know I'm fishing for answers here, but if you don't cast, you don't get a fish.....So, what's the story on the rear brakes. Did you replace the brake lines?

zodiacblueS

The rear brakes weren't touched other than bleeding. I'll pull the drums and see if the pistons are working like they should, but if I'm idling, the pedal feels right. Once I'm in gear they are hard. Getting better, semi functional, would never do in a panic stop, still not right.
Thanks

Allan R

By process of elimination, if only the BB and MC were really touched? It HAS to be one of those causing the problem, especially if they worked fine before the rebuild.

Try this:
Check the brake power booster by pumping on the brake pedal while the car isn't running. Continue pumping until you've "bled off" the vacuum from the booster.

Hold the pedal down while you start the car. The pedal should go down a bit more beneath your foot. If it doesn't, re-check the connection between the vacuum hose of the brake power and the engine's vacuum.

Oh, you were asking about the proportioning valve. Yes, it has moving parts and can wear out or malfunction. They are somewhat pricey. Maybe less at NAPA. In line Tube has the 71-77 for 85.00


I'm not going to suggest you try replacing it unless you have no choice.

Rickman48

And I believe there's adjustable proportioning valves available aftermarket that're a little cheaper, but come in handy if you've a big/little tire combination!

Allan R

Neat idea Rickman, his 73 is pretty much stock though; check out his web page - the car is mostly original and has an awesome history. It was in the family since new.

zodiacblueS

I did the pedal test, and there was no additional movement on starting the engine. The connections are new and sound from the manifold to the check valve. I can pick up a check valve for less than $10 at NAPA. I guess that's my next move. Do you know if it should have any sealant or lubricant on it when pushed into the booster itself? Seems like when I pulled it off before to check if it was holding, there was a lubricant, maybe lithium? on it. I am still confident Steve at brakeboosters did a good job on the rebuild itself. Hopefully this piece will help. Maybe a slow leak, or not a complete seal. It's close to working. But only if i sit in the garage and run the engine for a bit.
Thanks

Allan R

I wasn't suggesting that Steve didn't do a good job. From what you said earlier the BB is holding vacuum really well. The check valve should be ok, but what the heck, for 10.00 it's not going to break the bank. Won't hurt to try it. If you decide to use any lub on the hoses? I've used vaseline to make them fit easy, and it forms a pretty decent seal without causing any damage to rubber parts. You should have a spring clamp on the hose that attaches to the BB - do you have one on there?

zodiacblueS

I replaced ther stock spring clamps with band clamps and they are quite snug. Will keep you posted. Thanks.

MDchanic

I'm beginning to wonder whether maybe you're going about the troubleshooting bassackwards.

We know that your manifold vacuum is just a little bit low because of your cam.
Why not connect a known strong vacuum source to your booster and see how the system works with that connection, and then work from there?
In other words, if you've got an electrical vacuum pump (like for evacuating A/C systems), hook that up.
If you've got a hand pump, hook that up and pump till you hit about 20"hg.
If you don't have either one, then connect it to another car (spring for $10 for a long piece of hose).
If you do this, you won't be able to drive it, but you'll know right away if the booster feels right after moving a few feet in the driveway.

If all is well with this test, you can exclude all of the parts on the car, and focus on your vacuum level. Test the vacuum that makes the booster work right, and the vacuum you've got from your manifold.
You may need a vacuum pump in the car or a non-stock booster.

- Eric

Allan R

OK, I just read something on this site with another thread that made me go 'Hmmmmmm'. By ANY chance are the calipers on the wrong sides??? Bleeders should be at top of caliper. If this isn't the case, disregard. Not poking, just asking.