Brake help needed

[birdistheword]

So I replaced my booster and master cylinder about 2 years ago when I bought my 72 K10 and the truck sat in storage pretty much 95% of the last 2 years. Brake fluid will seep out of the front of the master cylinder lid while driving/braking. The peddle feels pretty soft while the truck is running and I have to hold it to the floor if I'm going over 35 to stop in a reasonable distance. I've bled the brakes about 100 times and don't see any air come out. And there are no leaks in the system. Any help would be greatly appreciated.

[Wrenchbender Ret]

Make sure the rear brakes are adjusted up good. Check to see if the emerg. brake cable are hanging up or are adjusted up too tight. If the emerg. brakes hold the shoes off the anchors it will cause a spongy pedal.

[birdistheword]

I'll have to check the rear brakes. I don't even have any of the e brake system on my truck at the moment. The brake pedal felt fine before the booster/master replacement other than the master was leaking into the booster and making the pedal slowly drop to the floor if I was at a stop light to long.

[luvbowties]

Originally Posted by birdistheword

So I replaced my booster and master cylinder about 2 years ago when I bought my 72 K10 and the truck sat in storage pretty much 95% of the last 2 years. Brake fluid will seep out of the front of the master cylinder lid while driving/braking. The peddle feels pretty soft while the truck is running and I have to hold it to the floor if I'm going over 35 to stop in a reasonable distance. I've bled the brakes about 100 times and don't see any air come out. And there are no leaks in the system. Any help would be greatly appreciated.

Sounds like the pumping action is pushing, at least partially, backwards and into the mc reservoir, on that half of the system[ "fluid will seep out of the front of the master cylinder lid while driving/braking"]--indicating a bad mc. I'd bet that if both lines were loose at the mc, there would be much less fluid exiting that one port when pedal is pumped just once, say. Be aware, jmo; but mc's are not so expensive if one would fix the problem and make it safe. Good luck with your fix.
Sam

[birdistheword]

Thanks I'll have to try that tomorrow.

[Mister-B]

Yeah, I'd recommend a new MC. They're only like $17 at oreilly. I chased all sorts of brake issues, and it ended up being two things. Bad rear cylinder and a bad MC. I started with the cylinders because one was leaking, but then I bled and bled and bled and bled and nothing worked until I replaced the MC

[birdistheword]

Well sure enough I did that test and the rear of the master cylinder stopped pushing out fluid before the front did. Got a new master and putting it on in the morning.

[LockDoc]

Originally Posted by birdistheword

Well sure enough I did that test and the rear of the master cylinder stopped pushing out fluid before the front did. Got a new master and putting it on in the morning.

It will be interesting to see where this goes. It is my understanding that this is normal for a disk/drum brake vehicle. The drum brakes take less fluid to operate than the disk brakes.

Info below is for the drum portion of the brakes only…

One thing I found on mine is when you put everything back together and adjust the brake shoes you can’t just adjust them out until they start rubbing and quit. You have to adjust them out as far as they will go and you cannot turn the drum at all. This centers the whole assembly in the drum. (I even tap around the drum as I tighten them to help them center themselves) Then you back them off until the drum just turns, and stop. If you only adjust them out until they start rubbing and quit, the first time you press on the brake pedal the brake shoe assemblies will center themselves and you will have too much clearance between the shoes and the brake drum.

I know it is a pain in the butt to try and hold the self adjusters off of the adjuster wheel while you back them off, but this is the only way I could get my shoes adjusted correctly. I always put 3 lug nuts on backwards to hold the drum on while doing this.

Bench bleeding The MC is also very important.

LockDoc

[68c10airstream]

Hi Erik, as a ase mechanic of 30 plus years a little trick that helps some of the time with a poor pedal is this, with the master cylinder and lines hooked up, remove the master from the booster and tilt the front (nose) of the master down with the lid removed, Now gently push the master plunger in and out a little and watch for bubbles! Usually when doing brake work/or dealing with a poor sealing master air will get trapped in the front of the master and can't get out because the front is pointed up. When i was working at the dealerships there was a very steep hill i would go to on my test drives and stop and exercise the brake pedal both pointed uphill and downhill to make sure all master cylinder air was removed! I suggested removing the master with the lid off so you could see the air bubbles and verify what was going on. Good luck, Brian F.

[birdistheword]

Thanks for the tips. Well I put a new master on after bleeding it and I still have the same issue. So once I get the new on and everything set I'm going to trying bleeding the brakes with the engine running. I was also told today that it could be my booster starting to go out but I would think that it would make the pedal stiff.

[luvbowties]

Originally Posted by birdistheword

Thanks for the tips. Well I put a new master on after bleeding it and I still have the same issue. So once I get the new on and everything set I'm going to trying bleeding the brakes with the engine running. I was also told today that it could be my booster starting to go out but I would think that it would make the pedal stiff.

Hey Bird, I'd follow LockDoc's method for adjusting your rear brakes as the next step. There's no better--or even close as good--way to adjust them imo. Sounds like the mc is good to go and should be ruled out now, as any source of troubles. And, if rear brakes aren't adjusted properly, you'll always have a low pedal. BTW: this next step costs zero other than your time, which is so enjoyable you shouldn't charge anything for it!!!

I was told years ago by tire and brake specialty shops that they always go straight to rear adjustment, whether rears are disc or drum, when customer's only complaint is a low pedal. BTW: Is your mc tilted downwards on your truck? ...or level?...or upwards?...as measured with horizontal, not as it relates to the booster.

One last thing: I don't see how a booster "going out" could cause the pedal to go to the floor. Unless, the mc and booster do not match: the booster pushrod should barely reach the mc piston(really about 1/8" clearance) if the booster's rod ends approx. flush with booster's "snout" AND mc's piston has no "pocket" made into it. If the mc has this recessed pocket(kind of a hole about 1" or so up into the mc's piston), then the booster's push rod should extend past its "snout" enough to reach up into this "hole" and nearly bottom out(again, approx. 1/8 inch clearance). **Look at pics of various MC's in Advance Auto Parts site to see the 2 mc pistons.**These clearances are when the engine is off and brake pedal is sitting freely. There is actually made a bullet-like spacer to fit up into the mc piston hole when the booster is mis-matched and has a too-short push rod of its own.

Attached is a pic of the 2 types of boosters--one with approx. flush rod and other with the extended rod that reaches up into a mc piston with the recess/hole.
Sam

[birdistheword]

Well I have the booster with the extended rod. Tried to bleed the brakes today with the engine running and got nowhere. It almost feels like I'm getting to much power assist if that makes any sense. I also measured the vacuum of the hose supplying the booster and it read 10-11 at idle but the engine does have a bit of a cam in it. Also noticed that my calipers look 500 years old. I'll probably try taking it to the do it yourself shop on base this week or weekend and putting it in the air to adjust the rears and check out my calipers.

[jocko]

bird,
a) when you originally replaced the master cylinder and booster, did you buy them as a single unit/part number replacement set or did you buy a booster and master cylinder separately?
b) as for the master cylinder you JUST bought - did you specify that it was for a truck with power brakes?

luvbowties is pointing out a critical thing here - the booster and master cylinder need to be matched when it comes to the intermediate rod. In my experience, I've only seen a DEEP master cylinder piston bore on manual brake trucks and I've only seen the shallow bore pistons on the power brake trucks. So, in other words, I've never seen a booster with a LONG intermediate rod - so, luvbowties picture #1 is a new one on me (looks like it would be made specifically for adding a booster to a non-power brake truck so that it properly mates to the manual brake master cylinder piston bore). But - the most important point here is that you need to make sure that your intermediate rod is the correct length. If you have a shallow bore master cylinder piston, then you need an intermediate rod like the one in luvbowties 2nd pic. If you have a deep bore master cylinder piston, then you need the one in the first pic.

If you didn't take note of whether or not you had the correct intermediate rod between mc and booster, it could be the cause of your problems. If you mis-matched a shallow bore mc piston with a long intermediate rod, your brakes would be dragging or the booster diaphragm would be compressed prematurely. If the opposite were true - deep bore manual master cylinder and shallow intermediate rod from the booster, depressing the pedal would likely result in almost no braking action because the short rod and the deep bore mc piston won't engage fully, or at all.

If you've had it apart a few times, then this is probably all obvious, but it is a common place for a hiccup to occur, especially when someone adds a booster to a manual brake truck (not the case for you though, you already had power brakes, I realize that).

[birdistheword]

I bought it as a whole already bolted together assembly from I think Autozone. The part I don't understand is that there is 2 different types of boosters that are sold at O'Reilly's but use the same master. I'm getting close to saying hell with it and taking it to a shop and tell them to just fix the P.O.S.

[swamp rat]

Originally Posted by birdistheword

I bought it as a whole already bolted together assembly from I think Autozone. The part I don't understand is that there is 2 different types of boosters that are sold at O'Reilly's but use the same master. I'm getting close to saying hell with it and taking it to a shop and tell them to just fix the P.O.S.

I feel your pain because i have been fighting my brakes every since i bought the new booster and master cylinder about 2 months ago. I get lots of fluid to the rear but next to nothing to the fronts, at first i thought it would be my proportioning valve needing reset but trying to re bench bleed the master cyl i'm beginning to think i may have a bad one. I don't know if i have miss matched parts ether but at least the intermediate rod that came with the booster is about .050 less than the space between the two..

[LostMy65]

This thread is another good thread.
I've spent the last few days in search for matching my manual master cylinder to a booster. From my research I'm finding that some of the older generation vehicles used the same Master Cylinder whether power or not - same depth bore. So in those cases, the longer intermediate rod on the booster would make sense.
My motivation is I don't want to have to bleed my brakes. I've found threads in the 67-72 forums where all they did was unmount their master cylinder from the firewall and replace it with a 67-72 booster.

[LostMy65]

I'm thinking of just making a rod to slip into the master cylinder like this:

[luvbowties]

Originally Posted by LostMy65

I'm thinking of just making a rod to slip into the master cylinder like this:

Hey, LM65, we've made a few just such "adapters" that worked plenty fine. Actually, we have found a couple of instances where one comes with purchase of a mc. Seems I recall 1--and only one--place where one was shown as a separate part number.

But u already see there is no need to bother hunting a source: they are so simple and need such non-critical dimensions. All needed are a bolt with an unthreaded portion(or section of steel rod), a hacksaw, a grinder, and a drill to create the pedal-end-divot. **DO NOT DARE USE A .38 SPECIAL CARTRIDGE, REGARDLESS OF SIMILARITY**

Luck with a successful outcome.
Sam

[LostMy65]

Originally Posted by luvbowties

Hey, LM65, we've made a few just such "adapters" that worked plenty fine. Actually, we have found a couple of instances where one comes with purchase of a mc. Seems I recall 1--and only one--place where one was shown as a separate part number.

But u already see there is no need to bother hunting a source: they are so simple and need such non-critical dimensions. All needed are a bolt with an unthreaded portion(or section of steel rod), a hacksaw, a grinder, and a drill to create the pedal-end-divot. **DO NOT DARE USE A .38 SPECIAL CARTRIDGE, REGARDLESS OF SIMILARITY**

Luck with a successful outcome.
Sam

Thanks.
I also found another post recommending the same thing.

Originally Posted by robnolimit

Here is a fix that we use from time to time. The bore wil be appx. .445". calculate/measure the difference/gap from the booster rod to the M/C piston bore bottom, and then cut a piece of a 7/16" bolt or cold rolled rod to fill the gap. It will be appx. 5/8" long. Slide the spacer into the master, check the fit, and go go go.

[Roadkill58]

Earlier this year I lost the brakes on my 71 Cheyenne.

Took the mc to all the local part stores including Napa and could not find one that matched.


I ordered one from Summit and got lucky a perfect match.


.

[birdistheword]

Well it turned out that I was just being an idiot and needed to adjust the rod going from the brake pedal to the booster.

[mechanicalman]

Originally Posted by birdistheword

So I replaced my booster and master cylinder about 2 years ago when I bought my 72 K10 and the truck sat in storage pretty much 95% of the last 2 years. Brake fluid will seep out of the front of the master cylinder lid while driving/braking. The peddle feels pretty soft while the truck is running and I have to hold it to the floor if I'm going over 35 to stop in a reasonable distance. I've bled the brakes about 100 times and don't see any air come out. And there are no leaks in the system. Any help would be greatly appreciated.

Are you still having problems? Does it always have air in the system when you try to bleed it?