Brake Bleeding Troubles

[pist0lpete]

Background: Baseline is factory style 68-72 chevelle front discs and rear drums that worked great along with an adjustable proportioning valve. I swapped to C5 vette brakes all the way around on my 67 Chevelle using Kore3 brackets and used calipers a couple months ago. Master is of unknown bore but would be factory for a 68-72 chevelle along with a single diaphram 11 inch booster. My guess is it is a 1" or 1-1/4" bore. Pads are Porterfield R4s which is fairly aggressive. Bleeding method has been a mixture of mityvac on the rear and speed bleeders on the front.

I have bled what seems like a couple gallons of fluid through the brake system and I still don't have the pedal feel I would like. As in the car doesn't even begin to stop until the pedal is just a few inches off the floor. Once the pads/rotors get warm at Autocross events the pedal travels even a a bit further but the hot brakes grab hard before the pedal bottoms out. No trouble locking up the tires at speeds below 35 mph when hot.



I realize I may need to step down to a 7/8" bore master to up my line pressure but won't that make my pedal travel problem worse since less fluid volume moves per inch of pedal travel? Are the aggressive pads which seem to lack in cold bite part of the problem? Any help is greatly appreciated.

[andrewb70]

Are you making enough vacuum for the booster to work properly?

Andrew

[fleming23]

Andrew has a good point certainly. Beyond that, in my 70 Chevelle, I found that the pads that came with my Wilwood D52 calipers left me feeling like I had pretty poor brake. I have since upgraded beyond that and had a bad pedal issue that was resolved by bench bleeding my master. This was in my 71 Vette with hydroboost but had symptoms exactly like you described - I had good brake but it was at the very bottom of the pedal on the race track.

[pist0lpete]

I don't believe the booster or vacuum available are the issue since the pedal feel was good before the caliper change. Though I admit I haven't put a vacuum gauge on the car I don't think an ls1 with a mild cam would be too low on vacuum.

[Apogee]

Just my $.02, but a mild LS1 should be creating great vacuum levels. Plus, if it were a low-vacuum situation, you would expect to have a high, hard pedal with a low brake torque scenario. That said, you should figure out your master cylinder bore size if for no other reason than to just know what it is. There's a huge difference between a 1" or 1-1/4" bore unit, so I have to assume that it's the 1" or something thereabouts.

It sounds like you have air in the system, hence the low pedal. Can you post up pictures of the front and rear calipers installed for our reference? When you lock up the brakes, are you locking up both the front and rear, or just one end?

Tobin
KORE3

[pist0lpete]

Tobin-

I appreciate you checking in on this thread.Is there an easy way to tell what bore my MC is? There is a pic of the booster MC setup below. My hunch is 1" as well. The brakes locked up rear first until I adjusted the proportioning valve and now the front locks up first. I admit my primary focus has been on bleeding the front which is easy with the speed bleeders. My guess is either they are causing me issues or the rear brakes need to be re-visited.



Edit: Here is a better pic of the calipers installed bleeders are at the highest point fyi.

[a67]

The only way to tell what size the M/C is is to measure it. May be able to do this without opening the system. Need to access the rear where the pushrod enters. Then measure across the bore, dial calipers are best, but a small 6" machinists rule can also be used.

Since this was a rear drum setup the M/C may have a residual pressure check valve in the rear brake port. If so it must be removed, otherwise the rear brakes will drag.

I've found that bench bleeding a disc brake M/C via the looped-back tubing into the reservoir doesn't work that well. The reason is the lack of residual pressure check valves. Stroking the M/C just moves the fluid and air back and forth between the bore & reservoir.

I've moved to the plugged port method of bench bleeding. Sometimes plastic plugs are included with the M/C, else make up two plugs from brake fittings that plug the front and rear ports. Then stroke the M/C with the nose pointed slightly downward. Note the bubbles that come up through the compensation ports.

Once they stop and the M/C locks up solid with a short stroke, maybe 1/32", it is bench bled. Mount on car, then remove a port plug and quickly install the line for that port. Then do the other line and finish bleeding the entire system,

Bob.

[pist0lpete]

Bob, I appreciate the insight. Starting from scratch bench bleeding the master may be a good idea. It was clearly bled before the brake swap but I could have managed to introduce air with all my bleeding. As for the residual pressure valve, my car was factory equipped with 4 wheel drums so my understanding is it would not have had the residual pressure valve installed from the factory.

[a67]

It is the drum brake M/C's that have the residual pressure check valves. Or at least should, many years ago the rebuilders would leave them out which caused braking issues. The wheel cylinders would weep fluid out and pull air in. A real mess until we figured it out.

Kinda' funny, walk into parts store and ask for a '67 Buick LaSabre M/C. Pull it out of the box and insert a straightened paper clip into each port. If it went in more then a 1/4", no valve. Hand it back and hit the next store.

Note # 15 & 16 in picture, this is from the '67 Pontiac FSM:



Bob.

[pist0lpete]

Thanks for the info Bob. I have a disc master so I don't think that should be an issue but I will take a look when I bench bleed.

[pist0lpete]

Bringing this thread back from the dead. Update is I installed a 67 Corvette 1"bore master and the pedal feel is only slightly better. Can't lockup the brakes at any speed cold but once hot they will lock up the tires at 50+ mph but still fairly low in the pedal travel. Also, I should mention, the prrevious master turned out to be a 1-1/4" bore.

Is there any chance I have an issue with the factory drum brake proportioning valve? Is there a spool in there that could be stuck?

[malibudave]

Is the pushed coming out of the vacuum booster going into the master cylinder long enough? If the push rod is too short, you may get a low pedal.

[pist0lpete]

Good tip, I actually think the booster rod may be sticking out too far. I read on another thread how a gentlemen had issues bleeding because of this. Going to look into adjusting my rod.

[pist0lpete]

Update, I have tried vacuum bleeding with the master unbolted from the booster and that did not seem to help. However, I have continued to bleed tapping on various parts of the lines and the pedal feel has improved slightly. I can at least now lock up the tires when the brakes are cold at lower speeds. At this point I am going to call the brakes good enough and move on to the other power tour thrash items. I feel there is still a small air bubble or possibly i just really need an even smaller bore master as the pedal travel is still more than I would like.

[minendrews68]

Look at the disc brake calipers. Often times there is a small air pocket at the top of the caliper and the only way to get rid of it is to take the caliper loose and tilt the caliper up where the line comes in. Don't take the caliper off of the disc, but this allows the bubble to find it's way to the top where it can leave via the brake line. Be sure to watch fingers while doing this as whoever is pumping the brakes doesn't see where your holding the caliper. Ask me how I know about this....

[Barrrf]

Update, I have tried vacuum bleeding with the master unbolted from the booster and that did not seem to help. However, I have continued to bleed tapping on various parts of the lines and the pedal feel has improved slightly. I can at least now lock up the tires when the brakes are cold at lower speeds. At this point I am going to call the brakes good enough and move on to the other power tour thrash items. I feel there is still a small air bubble or possibly i just really need an even smaller bore master as the pedal travel is still more than I would like.

Interesting I just happened on this thread. A couple of weeks ago I finished installing CTS-V (Gen 1) brakes on the rear. The front were installed last year. Bleeding the fronts took no time at all. But when I did the rears it took FOREVER. So much so I thought I was doing something wrong. I vacuum bled each caliper until I finally got fluid. Still no pedal. So I vacuum bled all the corners until I got fluid. No pedal. Then I did the ol' pump and hold method on all 4 corners until I finally started to get some pedal. I had to bleed nearly 2 bottles of fluid through until I could get a good pedal.

But when the car sits for a day I have to pump the pedal 2-3 times to get pressure again. I wonder if its the valve issue you spoke about earlier?

[pist0lpete]

Interesting I just happened on this thread. A couple of weeks ago I finished installing CTS-V (Gen 1) brakes on the rear. The front were installed last year. Bleeding the fronts took no time at all. But when I did the rears it took FOREVER. So much so I thought I was doing something wrong. I vacuum bled each caliper until I finally got fluid. Still no pedal. So I vacuum bled all the corners until I got fluid. No pedal. Then I did the ol' pump and hold method on all 4 corners until I finally started to get some pedal. I had to bleed nearly 2 bottles of fluid through until I could get a good pedal.

But when the car sits for a day I have to pump the pedal 2-3 times to get pressure again. I wonder if its the valve issue you spoke about earlier?

I feel like have bled at least two 32 oz bottles thru the system. Via the vacuum and the old press and hold method. Good to know I am not the only one however.

In regards to the check valve, that should only be present on the drum or drum disc master cylinders. What master are you running?

[65 Fastback]

Following.

[andrewb70]

Update, I have tried vacuum bleeding with the master unbolted from the booster and that did not seem to help. However, I have continued to bleed tapping on various parts of the lines and the pedal feel has improved slightly. I can at least now lock up the tires when the brakes are cold at lower speeds. At this point I am going to call the brakes good enough and move on to the other power tour thrash items. I feel there is still a small air bubble or possibly i just really need an even smaller bore master as the pedal travel is still more than I would like.

Decreasing MC bore size will increase pedal travel.

Andrew

[Barrrf]

I feel like have bled at least two 32 oz bottles thru the system. Via the vacuum and the old press and hold method. Good to know I am not the only one however.

In regards to the check valve, that should only be present on the drum or drum disc master cylinders. What master are you running?

Embarrassingly, I have no idea. I can only assume it was the stock MC that came with the car, drum/disc. I could ask the previous owner but everything he has previously told me has been false and his work on the car was less than stellar.