Problem building up brake pedal pressure....

I'd say it sounds like a problem in the stroke of the pedal or the lines going to the master are switched. Some of those older type boosters had adjustable rods also. Measure the mc depth, then measure the distance that the rod comes out of the booster. You could always disconnect the lines from the master, screw in your bleeders that come with the master to bleed it and stroke it in a vise to see how much fluid you are moving. One more question, was the master bleed correctly?

 

You can't just feel if the pedal is bottoming out as you pump the brake?
that last 1/4 " or so makes all the difference

This thread has helped me, I need to swap mc lines and maybe new rear calipers, mine are jy fresh

 

OK...I went over there after lunch and shot a couple pics and we piddled with it a while...Still not any better then we were...might be worse...but we'll get to that.

I did find out that the booster/master cylinder/proportioning valve came from Performance Online. The frame is a 72 Chevy with stock front disc brakes (rotors, calipers, lines). The rear disc kit came from Speedway, Supposedly the calipers are originaly intended for the front of a Chevelle.

Here's a box for a rear caliper with a part #




Now, before I get to deep in to what we tried today I'll show you what he's working with.

Brake pedal, the upper hole was the factory hole. He added the lower hole because it seemed to be where the angle of the rod would be the same as the angle of his booster.








Where rod comes through the firewall.







T-union for front brakes.





Front caliper.



Union in line heading towards the rear.





Another union.



Flexible line going from frame to rearend.









Gimme a minute and I'll get the rest of the pics up.

 

Some nice looking pipe bends there; I'd suggest the rubber lined clamps should be smaller to better hold the brake lines. I'd also use more of them to prevent the lines from getting fatigued with vibration. Too long an unsupported run isn't good.

 

This is how he was trying to bleed the rears last week when I was over there. He let the calipers pivot back on their lower pins so that the bleeders would point straight up. He used a block of wood and a C-clamp to hold the piston in. You could see the piston push against the wooden block but we weren't building up a pedal or getting any air out of the bleeder valves.



Here are how the calipers look as installed.











We started on the theory of weather or not the pedal was allowing the piston in the master cylinder get a full stroke. We unbolted the master cylinder and moved it to the side so we could see the rod that comes out of the booster. When the brake pedal is depressed the rod comes out of the booster 1.5"



Back of the M/C.



Then we went ahead and took the master cylinder back off the truck and put it in a vise to measure the stroke of the piston. We measured it at 1 & 1/8". So if our measurements are even close, the pedal stroke should be sufficient. The master cylinder was re-bled while in the vise and re-installed.



After reinstalling the M/C the brake pedal seemed better (at least to me anyway). We decided to try to rebleed the rear brakes. We kept getting fluid but no air...And the pedal isn't dropping when the valves are cracked...and now for some reason we aren't seeing the pistons move and the pads aren't even moving out to the rotors!....So,...we go back to the front to see if we've gotten air in the front lines with all of our removing and reinstalling of parts (m/c). We were able to get some air out of the front bleeder valves (but not a lot). The pedal seemed to be improving...right up until the resevoir of the m/c went dry....We filled it back up and tried using a vacuum pump on the calipers and then bled them again pumping the pedal and cracking the bleeders. We finally felt we had the fronts back to where they need to be...but the back still isn't cooperating. Can't seem to get any air out of the bleeder valves...the pistons aren't moving now (they did last week), the pedal doesn't drop when the rear bleeders are cracked.

At this point he topped the master cylinder with fluid, cracked the bleeders open and decided it was a good time to go out for a beer...no idea how it looked when he got back home.

So...is the combination of parts from two different suppliers causing an issue? Is it really just as simple as air in the lines? I've never seen brakes this hard to get all the air out of, and yes I've done cars with all new components before (they weren't this problematic). If it's really only air, is there a better pump system then whatever he bought over the counter at the parts store to help eliminate it?

Any HAMBers in the Chattanooga area feel up to the challenge...At this point he's more then happy to pay someone to fix this!

Thanks in advance for any info, suggestions or ponderings that might lead him down the right road to straightening this out.

 

When the master went dry, did you bleed it again? I didn't see any bleed pipes on your master when it was in the vise? Masters often come with a set of plastic nuts and a short length of hose to bleed the master back into it's own reservoir. I use the hoses to blled tha master, then leave 'em on until it's on the car, then quickly change the plastic fittings for the vehicle ones.

Here's what I would do:

Bleed the master on the vehicle, using the technique where you crack open each fitting on the master, get your buddy to push the pedal to the floor and hold it there until you close the fittings again. then have him gently release the pedal. repeat until no air comes out around the fittings. DON"T let the reservoir go dry or you will have to start all over again. Now bleed the proportioning valve using the same method. Now do the front brakes, again having your buddy push the pedal down while you crack the bleeder open. Close it and have him release the pedal. repeat as req'd.Then do the same to the rears.

 

I think you got a air trap in the hose from frame to rear axle,the pic shows it making a vertical loop. That is a real good way to trap a air bubble. Try tying it down so it lays horizontal and then re-bleed.

 

I'd gladly come and help, but I'm about 1800 miles away!

 

According to what I can find the front calipers are meant for "Chevy/GM/GMC/Cad Truck 79-96".
Rear caliper is from a mid-size GM, early 70's, Regal, Camaro, Century etc. Even the big block cars used this caliper...in front.

What was the bore size of the master cylinder and the OEM application?

The combination valve. What's the OEM application on that?
Is that a brake warning switch wired in? Is the warning light hooked up and LIT when your bleeding? (Key on.)
Whats the original use for the port on the top/front?
Some of those combo valves need to be recentered as you bleed the brakes. They can be a bit weird...


BTW...looks like he'll have a good pedal every time the rearend hits the bump stops! I think the flex hoses will get pinched! (I'm sure he'll change the position...just messing around with ya...)

NOT knocking the job he's done mind you. That truck is built really NICE.
Very clean workmanship! Nice to see.

 

Has this been eliminated as a possibility?

 

No idea on what the oem application of the m/c or combination valve would be. Also not sure on what the bore would be...I know he had tried to slide a couple different "straight edges" in it for measuring and they wouldn't quite fit...maybe we can go back and look at those pieces to get a "close" idea of the bore. I didn't think about it while it was off...we were focused on checking the stroke.

That is a sensor for a brake warning light...it's not currently hooked up (the instrument cluster isn't installed yet). I'm not sure how he was reading it...either with a meter or...I really don't know. I do know at different points of this ordeal he did say it was indicating that it would be lit...(I assume this meant the valve was tripped and the springs had to be recentered).

The port on the top front that is plugged is an alternate port for the front brake lines if you chose not to use the lower port. ( at least I think that's what I read in the instructions).

I got on him today about the bumpstop/brake line correlation back there...He's on it. I think he's going to pull the fuel tank and c-notch the frame (and move the lines) which should correct any interference issues....Of course it'll never be an issue if he doesn't figure out the brakes.

Edit: I just skimmed back through the pics again and saw the one I think you were refering to with the bumpstop/brake line....It looks WAY worse in that pic then it really is. I did get on him today because the corner of one of the stops looked like it would come close to hitting the line during compression...We'll get him straightened out. ;-)

 

No idea I'm afraid...The booster/master/proportioning valve came from one vendor, while the rear disc kit came from another.

Yes...he's now quite aware that mixing parts from different vendors can leave him with...well...what he's got now.

I'm beginning to think he needs to place a call Monday to the supplier of the booster/master/proportioning valve to see if they can provide him with accurate specs on the size bore and stroke of the m/c. Might also ask them what rear calipers (bore/capacity?) they recommend to work with their parts.

 

Seems like at one point he had tried doing just what you're suggesting...Of course, I may just be imagining that he said he had done that. We'll be sure to give it a shot the next time around.

 

Hmmmm...this is interesting!
Check out the 5th master cylinder down. Aftermarket Corvette stuff.
Looks just like what your friend has!
Read the blurb...and notice how your friends is configured.

http://www.camarobrakeparts.com/products.asp?cat=11

I dunno...just passing on what I came across while looking around to try to help you guys!

 

I see the same thing that you saw...Upper ports for discs lower ports for drum...I don't think his has a lower/upper port for the rear line..Do you think moving the front one will affect how the rear works?...just thinking out loud. His instructions had a diagram of the valve (including the internals). If I get over there tomorrow I'll see if I can pick it up and scan it.

Thanks to all that have checked in with suggestions...I know somebody will strike Gold here sooner or later.

 

Sometimes those thick copper seals shown in your picture of the rear caliper restrict flow to the caliper. There is a groove that has to line up with the hole in the end of the bolted on flexible line. The thick ones usually come with rebuilt parts and I've had to toss them occasionally. Try a thin set.

 

I don't know...

MOPARSLED suggested that the problem may be farther upstream than the rear calipers.
Perhaps getting a solid pedal when the rear flexline is pinched is just a symptom of something else.
Like was said..."If the rear system has no way to displace fluid (piston moves) and the fluid can't compress, how's he still pumping?"
Maybe it's just masking the real problem which is somewhere else!
I mean...your friend has replaced everything else at least once AND gets no air from the rear lines when bleeding!!!!

It's gotta be a basic design/hookup issue or else its in the parts he hasn't replaced. (obviously)
If I read right thats only the booster or the combo valve!
Combo valve hookup...especially IF you can buy one with two configurations built right in...might be it.
I don't know.

If it were MY truck and I was home alone in the garage, I'd first make sure the volume of fluid needed for ALL the calipers was actually available from this model master cylinder, and IF that proves to be the case, as a test I'd bypass the Combo valve with a couple of brass adapters and see what happens.
I figure (could be wrong!) that a good pedal then means the combo valve is faulty or more likely plumbed wrong...still no pedal means its in the Master Cylinder itself or the pedal ratio/booster hookup.
But I'm no expert...

In the mean time...ECIGUY offered his help on page 2 and he is!

I would SERIOUSLY consider taking him up on his offer seeing as how your friend has mixed components from several companies.
ECIGUY designs and manufactures brake systems.
Who else here could possibly be as qualified to guide you to the source of the problem?

We have the desire...but he has the credentials!

 

We did pilfer another proportioning valve that was known to be good from another vehicle...It didn't help our situation.

We actually have discussed bypassing the valve more then once...just haven't stepped up and tried it yet though.

This is actually the third master cylinder he's tried on the truck....still doesn't mean it's not the issue though.

I went back and read the post from ECIGUY (3rd page, post #50 for my lurking brakeless friend reading this). Sounds like my bud needs to call Performance Online tomorrow and find out the exact specs for the parts he bought from them, and then give ECIGUY a call.

Thanks again for all the input guys!

 

Everything you've described happened to me, as I posted previously. My problem turned out to be bad rear calipers (defective rebuilds). Maybe you are having the same problem?

 

If you had that ssbc pressure gauge you could just attach it further and further upstream from the caliper till you get good pressure and then you'd know where the restriction is plus you'd know exactly how much line pressure you're getting at all 4 calipers so you know its correct.

 

This gave me a lot of grief, mine was mopar but chev may be different.

 

my son just had this problem with a car (66 Acadian) he is building. He put on an aftermarket master cylinder and then tried to bleed the brakes (disc/drum) and they would not bleed properly. It turns out, the rod from the brake pedal to the master cylinder was too short. Made a longer rod and problem was fixed!

 

i secong the air being traped in that loop at the rear, it would be very hard to get the air out of there, in fact i dont think you can.

 

When I spoke to him earlier, he said that he was going to jack up the frame so that the rear axle could sag (possibly unbolt the coil springs from either the frame or trailing arms). He was either going to try gravity bleeding or using his vacuum pump on the rears again. Last week we had actually cracked the line open on either side of that flexible line in an effort to get out any air...got nothing but fluid.

In the meantime he's been given his homework assignment of finding out exactly what the specs are on the master cylinder.

Thanks again to all that have stepped up with suggestions!

 

Go to post #50, do as told, solve problem. Ralph is the Sulton of Stop. He does this for a living - not as a hobby.

 

I disagree. Brake fluid is viscous enough to push an air bubble down hill through a brake line. I use a brake bleed bottle that has a clear pipe and I often see bubbles traveling downhill. I always end up with a firm pedal....and even braking.

 

I was mowing my grass earlier and my bud called and left me a message...sounded something like "I'VE GOT BRAKES!"

Anyway I called him and asked what he did...and he didn't really know...seriously. Someone he goes to church with that piddles with cars had offered to come by and have a look at it. Mumbled something about the proportioning valve...Apparently we should have done more praying and less cussin.

Thanks to all that stepped in with advice and suggestions. Wish I could be a bit more clear about what the final solution was...He was just so excited to have pedal pressure that I wasn't getting a lot out of him. As soon as he calms down a bit I'll see if I can't step back in with something a little more concrete concerning the solution.

Thanks

 

Sweeet! Do let us know what the fix was.

 

After 5 pages, I'd love to know the ending to the story!

 

as stated before make sure the calipers are mounted on the correct side.left on left right on right,bleeder screws up !!!!!!! ive seen this happen more than a few times....