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Old 04-11-2018, 09:18 PM   #1
Kevin Mac
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PWR brake pedal slow to return

47 chevy with 383, mild cam, the brake pedal is pretty slow to return up. If I come to a roundabout and just slow a little the brakes drag until the pedal comes up. It also does the same from a stop light. I know that low vac. will cause only having a good pedal once or twice but will it also cause the pedal to return slow and brakes drag? It looks to be a common booster MC combo frame mounted. I didn't install it so not sure what brand. I added a vac. cannister to help with more vac volume but made little to no difference. Before you ask how much vac. I can't remember when I checked it last year what it was. I am thinking around 12 inches if my memory serves me right.
Going out to shop and work on truck, will check back in a bit.

Thanks Mac
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Old 04-12-2018, 09:53 AM   #2
dsraven
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Re: PWR brake pedal slow to return

a better description of your system would help but if the pedal is slow to return then the first thing I would check is the pedal pivot to ensure it is free turning. disconnect the pushrod to the pedal and see if it swings easily and returns to the top without dragging. next up I would check all the linkages and pivots to ensure nothing is binding or rubbing on something that may move with body flex. next check the pushrod free play at the master cylinder, you need to have free play so the piston in the master cylinder can travel back all the way and allow the fluid to return to the res, especially if you notice the brakes only drag after they have been heated up. with not enough free play the fluid in the system heats up, expands, and then the brakes drag because the fluid pressure can't escape back to the reservoir. if you have a bunch of linkages before the master cylinder then it could be that theweight of all the linkage and pivots etc is hard for the master cyl to overcome. a simple return spring on the linkage may fix the issue.
is your system a frame mounted system or a firewall mounted system? disc all around or disc front drum rear? are you running residual valves in the lines? do you have free play in the pushrod at the master cylinder?
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Old 04-12-2018, 10:17 AM   #3
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Re: PWR brake pedal slow to return

sorry, looked right past the part where you said frame mounted.
the vac should have no effect on the brakes dragging if the booster is working properly but you could disconnect and plug the vacuum from the engine and see if that helps. that would basically put you as if your truck had standard brakes. the pedal effort may be more than standard brakes actually, depending on what master cylinder is in there.
there is a valve in the booster that balances the vacuum on both sides of the diaphragm inside the booster, when you step on the brake pedal the valve opens and admits atmosheric pressure to the rear side of the diaphragm so the front side has a negative pressure (vacuum) and this assists the driver to push the pedal forward. if the valve inside the booster is shot or the atmospheric vent is plugged then I suppose it could delay the return of the pedal, depending on whats going on in there. this is why it is important to have free moving components because a little pressure against the valve in the booster could cause you no end of trouble. if you disconnect and plug the engine vacuum and try it this eliminates the booster valve so if the problem persists then it isn't the boosters fault. remember that the master cylinder used for a power brake unit will likely be harder to push than a regular standard brake master cylinder, without the aid of the booster, so be prepared to push harder on the pedal if you disconnect the vac line. this is because a power brake master cylinder usually has a larger bore size so the effort goes up in order to push the same pressure. also, remember to put some sort of filter over the vacuum hole in the booster so no dirt gets in while test driving. even a coffee filter taped over the hole would work. don't plug the hole fully, it needs to breath for the test to work properly.
here is a youtube on how the power brake booster works. it's british but explains it well. first one I came to on google so if you wanted to research more, have at it.
https://www.youtube.com/watch?v=0sj5d48mcss

here is how to do a test of the booster. some tools required but not gonna break the bank (harbor freight?)
http://ssbrakes.com/attachment/85569...0Diagnosis.pdf

the first thing I would check is the fee movement of the linkages, including the pivots.
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Old 04-12-2018, 10:18 AM   #4
jweb
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Re: PWR brake pedal slow to return

Here's what https://info.mpbrakes.com/faqs says about vacuum:

For a power booster to function properly you will need at least 18" of vacuum at idle in park. Anything lower will give you a hard pedal.

You need to have residual valves since your master is under the floor: http://mbmbrakes.com/proper-brake-system-plumbing/

Could also be a booster pin adjustment: https://techtalk.mpbrakes.com/how-to...r-cylinder-gap
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Old 04-12-2018, 10:23 AM   #5
dsraven
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Re: PWR brake pedal slow to return

here is a little youtube (a bit long winded) that explains the brake system in laymans terms. could ber helpful if you can spare a couple minutes. not a gm system but the same principles.
https://www.youtube.com/watch?v=I4Ka4__ws1E
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Old 04-12-2018, 10:31 AM   #6
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Re: PWR brake pedal slow to return

the other thing to remember is that the power booster under the floor is usually smaller so they may have 2 diaphragms in there to get the same square inches of working area as a larger booster. that compounds the likelyhood of having mechanical problems. another reason why some guys with the frame mount brake units are using the hydroboost booster, they are physically smaller and give great boost no matter what engine or vacuum is available. (no, I'm not a hydroboost salesman, lol).
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Old 04-12-2018, 04:01 PM   #7
Kevin Mac
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Re: PWR brake pedal slow to return

Thanks everyone. I will check on valves going to front. It is disc/drum setup. How can you tell if it's dual diaphragm? I will also check for binding of any linkage. It might just need a small spring. It will act the same on cold start up( when vac is low) I can pull it up with my foot and brake release quick. ��
Mac
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Old 04-12-2018, 05:36 PM   #8
dsraven
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Re: PWR brake pedal slow to return

if the problem doesn't just exist when the system is hot, like the brake fluid is physically hot from using the brakes, then I would say the pedal to master cylinder pushrod adjustment is probably fine. not saying don't check it but it is probably close to spec. when you say the issue goes away if you pull up on the pedal that makes me think there is a linkage binding or a return spring missing somewhere or the physical weight of all the linkage is overcoming the return spring so the pedal is always slightly pressed down and not allowing the master cylinder pistons to return all the way. sort of like holding your foot slightly on the brake all the time. my suggestion is to go under the truck and start at the brake booster. push backwards on the linkage (like you are releasing the brake pedal pressure) and see if the linkage moves back. if it does then there is a problem and you just need to keep working your way back towards the pedal to find where the problem spot is. also, it could simply be a binding pivot point right at the brake booster in the bellcrank fulcrum so make sure to start there.
as far as the dual diaphragm booster, it usually has a 2 stage cannnister section, like it is one size around and then bumps up to be a little bit larger around towards the pedal input end. they are usually longer, front to rear, because there is physically more stuff inside. just google it.
http://www.tpub.com/basae/176.htm
the drum brake master cylinders may have a residual valve built into the outlet port of the master cylinder (which is removable) but the disc brake masters shouldn't. some guys have had trouble when they use a master from something and then use a res valve as well and find they have doubled up on the res valves. you kinda gotta know what you have. if the master cylinder was originally for use with drum brakes it may have a res valve because drum brakes keep a little bit of pressure in the system to keep the wheel cylinder cups pressed out against the bores of the wheel cylinders so they don't leak and also so air can't get into the system. this can cause the disc brakes to drag if the drum brake master was used for a disc brake setup. the res valves are removable. just google it. they can be seen from the port where the brake line connects to the master-usually.
https://www.freeasestudyguides.com/r...eck-valve.html
does that help answer anything other than how many letters can be put into a response, haha?
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Old 04-12-2018, 10:41 PM   #9
Kevin Mac
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Re: PWR brake pedal slow to return

Thanks, I hope you didn't have to type all that on a flip phone. I didn't get a chance to get in the shop tonight, nor will I this weekend, but I will investagate when I have it up in the air changing spindles. The master appears to be as aftermarket unit with booster. I just didn't know how common it would be to have a return spring on the pedal. I will check for binding before I spring it. Thanks so much for taking the time to help.
Mac
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