A little help with my under-performing disc brake system?

[FLYNAVY53]

The car is a 1972 C20 Suburban. Factory it came with discs/drums. I have gutted the brake system completely. The only original part is the pedal itself. My issue is that I have a perfectly firm pedal, but the performance is underwhelming. No matter how hard you jump on the pedal, you can't lock the brakes up. It also takes what feels like an unnecessary amount of effort just to keep the truck from creeping forward at a stop light with the transmission in drive. Below are the specifics of the system. Any help would be greatly appreciated!!

Factory pedal ratio: approximately 5.6:1 (14 inches/ 2.5 inches)
Vacuum booster: factory replacement
Master cylinder bore: 1 1/8 Baer Remaster with pushrod depth set per Baer specs (.005" if clearance)
Brake line: 3/16 steel with braided stainless at all 4 calipers. -3 AN fittings throughout
Front Caliper: Wilwood D52 with 6.28" piston area and Wilwood BP-10 pads
Rear Caliper: Wilwood D52 with 2.46" piston area and Wilwood BP-10 pads
Fluid: DOT 5

[dontlifttoshift]

Is the booster working? On paper that all looks great, that setup should work quite well. BP10 isn't my favorite pad but with a factory booster they should be fine. Honestly, the stock stuff works quite well on those trucks (C20)

The fighting the pedal at the stoplight leads me to booster not functioning. How much vacuum do you have? Get in the truck, pump the pedal 3 or 4 times, hold your foot on the pedal and start the truck. Pedal drop? If not, junk booster. Another test, disconnect the vacuum line to the booster and drive, does it feel any different?

[FLYNAVY53]

Thanks Donny:

Showing 14-15 inches on vacuum at idle

Pumped the pedal, started the truck, pedal drop 1/2 - 1 inch.....nothing close to what modern cars do, but it did drop a bit

Disconnected the vacuum line, put the truck in drive, and the truck jumped forward with foot on the brake.....took everything I had to get it stopped before I rear ended my neighbors jeep. It would be completely undriveable without the vacuum line attached....100% pedal effort to stop from a 5 mph roll.

Any thoughts?

Thanks again,
Greg

[ULTM8Z]

Admittedly without having done the calculations, I'd be poking at the 1 1/8" bore MC. You could try dropping to a 1"... that would increase pedal travel a little, but would decrease effort to get the same braking force.

[dontlifttoshift]

Boooo. Little low on vacuum but not enough to make it undriveable. Sounds like the booster passes.

How long has this system been on the truck? Did you bed the pads in? What are your line pressures?

DOT 5 sucks. No one should use it on anything that actually drives.

- - - Updated - - -


Admittedly without having done the calculations, I'd be poking at the 1 1/8" bore MC. You could try dropping to a 1"... that would increase pedal travel a little, but would decrease effort to get the same braking force.

Stock bore is 1.25 with similar piston size in the front, look at the pedal ratio, with a booster. I would expect a low pedal with all that caliper but it sounds fine.

[oleyeller]

It sounds to me like your proportioning valve has shifted, and is blocking the front brakes from working. Ahhhh, You have a Baer Remaster...... I had a similar issue with a new Remaster, when I had help with someone pumping the brake pedal to bleed the brakes, when a rear bleeder was opened, the pedal went to the floor, when the pedal was pumped up and the front bleeder screw was opened the pedal went half way and stopped. It is a dual master so if it thinks there is a leak in one or the other chambers, the other takes the load. When this happened to me, I returned the master cylinder to Baer and they replaced it. New one works great. I struggled for weeks thinking I had air in the system or some booster issues. Must have been some internal leak causing the issue. Baer was great to work with and took care of me.

[FLYNAVY53]

System has been on the truck a little over a year....been bled more times than I can count. That being said, you may be onto something with bedding the pads....I don't know that I ever properly did that. The truck has a 6.0L LS with Holley fuel injection and a few other parts....after I finished building it, there were numerous bugs to work out over the course of a month or so, and I don't believe I ever went out and properly did that. You think I could have ruined the pads and thats why there is no real "bite" to the brakes?

Not sure about line pressure....that was kind of my next step if nobody on here saw anything glaring....I assume that entails just screwing a pressure gauge into the bleeder port on each caliper and checking the pressure?

Would love more info on the DOT 5.....I enjoy the fact that it doesn't eat paint, but I've heard positives and negatives.

[FLYNAVY53]

It sounds to me like your proportioning valve has shifted, and is blocking the front brakes from working. Ahhhh, You have a Baer Remaster...... I had a similar issue with a new Remaster, when I had help with someone pumping the brake pedal to bleed the brakes, when a rear bleeder was opened, the pedal went to the floor, when the pedal was pumped up and the front bleeder screw was opened the pedal went half way and stopped. It is a dual master so if it thinks there is a leak in one or the other chambers, the other takes the load. When this happened to me, I returned the master cylinder to Baer and they replaced it. New one works great. I struggled for weeks thinking I had air in the system or some booster issues. Must have been some internal leak causing the issue. Baer was great to work with and took care of me.

I can say for certain that its not the prop valve.....I was running the bolt on Baer prop valve and removed it to verify it wasn't the issue. Brakes are exactly the same with or without. As for an internal failure of the master cylinder, I kind of doubt it. I've bled this system at least a dozen times....both the old school method and using 5-10 psi to push down from the master....no issues getting a good firm pedal with either method.

[dontlifttoshift]

There are plenty of threads and info about DOT5, no need to muck up this thread.

Maybe, I would pull all the pads and scuff them with 80 grit, same with the rotors. Then go out and bed them in, 60 to 30 mph moderately until they start to stink, then a couple more times hard braking. Go back home, try not to stop on your way back, park it in the driveway and let them cool. See what that does.

After that, we need line pressures.

[FLYNAVY53]

Fair enough....Ill do a little more research on the fluid....

Ill mess with the pads/rotors and bedding them this afternoon.

I just ordered a pressure gauge from Summit....of course the Wilwood calipers have a different thread pitch than anything factory.... should be here tomorrow.

Thanks again for the continued assistance...I thought I had this system pretty well researched when I chose the parts I did for a street truck/daily driver. Its been frustrating that the brakes work well enough to drive, but they just don't have that initial bite that they should, or the ability to lock up the fronts.

[vette427-sbc]

FWIW I recently put a new wilwood system in a 69 nova with the BP10 pads. First drive I was nervous that we spec'd something wrong because it did need alot of pedal to stop the car. After the bedding process there was a HUGE difference in brake performance. The BP10's faded earlier than I would have expected, but stopping a full street trim big block nova from 100+ with 11" rotors maybe Im expecting too much

[FLYNAVY53]

Thanks Chris....hopefully that makes a difference

[Peter Mc Mahon]

Any updates on this?

[Chad-1stGen]

How many miles on the system?

I often run BP-20's for street and autocross and they take a fair amount of pedal effort to achieve lockup when everything is working properly. That extra effort is one of the reasons I like them. Anyhow, whenever swapping a new set of pads and following Wilwood's bedding instructions they work great. However, when I change the rotors and pads, then even after bedding them in they don't work 100%. It usually takes a couple of autocross laps or a fair amount of street driving until they feel good again.

Thats my long winded way of saying bedding the pads is a good idea. And critical with new rotors.

[FLYNAVY53]

Any updates on this?

Unfortunately not.....I was out of the country for a bit and just got back...need to get the truck out of storage and start messing with it again. I did get a Wilwood pressure gage so that I can check the pressure at each caliper and go from there. More to follow.

[FLYNAVY53]

Thought I should come back and update this thread as I did eventually get the issues sorted. I contacted Wilwood about switching to a more aggressive pad compound, and they said that would work, but it would also result in faster pad and rotor wear. The tech made a point of including that they recommend a minimum of 18 in. Hg at idle (as stated above) and an 8 in. DUAL DIAPHRAGM BOOSTER...

I re-verified that I was only pulling 15 in Hg at idle....not ideal. Then I started doing some research on single vs. dual diaphragm boosters. The math to figure out how much "boost" the system is applying is:

(vacuum at idle)x(booster diameter)x(number of diaphragms)

So based on Wilwood's recommendation of 18x8x2, you're looking at 288 "units of boost?"....not sure what that boils down to as far as units. I then did some research, and realized that the '72 trucks came with a SINGLE DIAPHRAGM booster because with the stock disc/drum set up, you dont need/want as much boost with the rear drums. So running the math on my truck as it sat....15x11x1+165 "units of boost". Well below Wilwood's minimum recommendation for a power brake system. On doing some more research, I quickly realized that the '73 trucks, with the 4 wheel disc brakes, came standard with an 11 inch DUAL DIAPHRAGM booster...part #2232NB at Summit Racing for $145.99.

So even with my lower vacuum at idle: 15x11x2 puts me at 330 "units of boost" which I was hoping would alleviate my lackluster braking performance and excessive required pedal force.

The bracketry wasn't exactly the same on the new booster, but with some slight modification to the new bracket, and piecing together a new linkage for the push rod, I had the new booster buttoned up quickly.

Initially, the pedal sat a lot higher, but I had purposely left a fair amount of adjustment in the rod, so I was able to adjust the pedal down towards the fire wall back to the factory position. With the dual diaphragm booster, the pedal feel is significantly different...very soft, like a modern car. Overall, I was 90% satisfied with the change. I decided to also throw Wilwood's Polymatrix E-compound pads on the truck. Those pads have a slightly higher coefficient of friction across the temp. range.

The combiner's nation of the booster and the pads completely solved the prior issues. Pedal force for a given amount of breaking is right in line with my 2016 4Runner. In a panic stop from 60, it brakes straight, with good pedal modulation, and will just barely lock up- the fronts without locking up the rears. On a dry road, it stops just like a modern truck. Even my wife commented on how much easier it is to drive. In the end, it took a little effort and ass-pain, but I'm quite happy with the performance of the brakes now!

[dontlifttoshift]

Glad you got it where you like it.