New front brakes won't lock up/big tires the problem?

Discussion in 'Axles + Brakes' started by gordonm1, Jan 10, 2019 at 3:36 PM.

  1. gordonm1

    gordonm1 Newbie

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    Newby here and have been reading the brake threads and trying to relate to my vehicle situation.

    My new front brakes won't lock up on dry pavement but they will on gravel. This has been an issue since before the brake work. The pedal travels a bit further than my other cars. My main diversion from stock configuration is going from tire size 225/75/16 to 245/85/16. I don't see much talk about larger tires on the brake threads.

    I wonder if the tire size increase is too much for my cheap W/T brakes?

    I might have even blown my master cylinder seals trying to get the fronts to lock up because during testing it started getting wet between the booster and master so I replaced with a rebuilt master.

    Truck is 1991 K1500 W/T standard cab (probably tiny cheap brakes). The booster is 9" diameter. The truck is OBD1 and has the RWAL brakes in the rear.

    I read the sticky'd thread above about brake upgrades and saw light duty 3/4 ton front calipers can be used in my situation. Would this help any?

    When shopping at autozone they showed a larger diameter booster (10-1/2") that is supposed to fit. Will it work or help any? I am a bit concerned more boost might blow my master cylinder seals again. I had a 20% off and free shipping coupon and the 10-1/2" is also $40 less than the 9" so I ordered the 10-1/2" already.

    I would pull my new 1/2 ton calipers and do the 3/4 ton upgrade next if it helps a lot. Then try the booster.

    Any help is appreciated...
     
  2. Jared Jackson

    Jared Jackson I'm Awesome

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    When you put on larger tires, you reduce the effectiveness of your brakes and your ability to brake as quickly. Newtons laws and such...

    Why is your goal to lock your brakes up?

    Just because you have a W/T, doesn't mean you have crappy brakes. They are probably the 368's... However, your factory parts were designed to run with factory sized tires... when you increase the size, you will decrease their effectiveness (see comment above).

    If your master was leaking, it's a good thing you replaced it. However, if your booster was soaked... you need to probably replace that too! I would have to read up myself on your available "stock" brake upgrades. Running a bigger booster would give you more diaphragm and therefore, more capacity for stored "boost", but I can't see it having an effect on your MC seals... Someone else may be able to chime in on your brake mods and their effectiveness.

    However, I can recommend grabbing some flat black Rust-Oleum paint and coating your booster before installing it. Looks a lot better and will help prevent corrosion.
     
  3. gordonm1

    gordonm1 Newbie

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    My existing 9" booster is double diaphragm and the 10-1/2" I have ordered is also double diaphragm so based on online research charts, this larger booster should increase my braking power a little.

    The 3/4 ton (light duty) calipers are supposed to fit but they will use more fluid and thus brake pedal travel so with my low pedal I would only do this with a master cylinder upgrade to the NBS or whatever which sounds like it raises the pedal some while having a larger diameter piston to flow more brake fluid.

    I read the NBS cylinder has a larger threaded hole for one of the brake lines so one brake line might be larger to allow more fluid flow to the front or the rear.
     
  4. Steve A

    Steve A I'm Awesome

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    I think you'll find that the "light duty" 3/4T calipers (7200 GVW) are probably the same as what you have already. IIRC the 8600 GVW calipers have a piston just under a 1/4" larger diameter and may be what you want instead. Use the 370 pads as well. Not sure all this will fit a '91 model but is what supposedly went on '96-'00 police Tahoes.
    Also, I don't think you'll find a big difference in Outside Diameter between 225/75/16 and 245/85/16 tires so there shouldn't be enough leverage change to make a lot of difference there.
     
    Last edited: Jan 14, 2019 at 2:41 PM
  5. Gibson

    Gibson Newbie

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    Not an issue, the pads in a disc system are already touching/dragging on the rotors lightly,, there is almost zero fluid "movement", what mainly happens is only the "transfer of pressure" within the fluid column.
    In a disc/drum set-up, if everything is adjusted and operating properly, with the rotors having no run-out, and the drums not being scored, somewhere ~90% of your pedal travel is being used-up in moving fluid to just get the rear wheel cylinders to push the shoes against the drums, before any braking action takes place.
    And you would be surprised at the amount of pedal travel that is used in a hard brake application in overcoming the expansion of the rubber hoses, the distortion of the rear brake shoes, and the flex of the front calipers.
     
  6. gordonm1

    gordonm1 Newbie

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    Thanks for the replies folks.

    I have new flexible hoses to replace my 27 yo original ones and the larger 10-1/2" booster is in the mail. I can still return the booster but I think it will provide 15-20% more brake power/ pressure. I acknowledge all the mentioned travel in the rear but it may equalize pressure with the fronts eventually. That sounds like a win for my front brake needs.

    I don't want to buy any more parts like calipers and pads and master cylinder.

    I will be adjusting my rear brakes and trying to bleed the dumpoff valve with my sisters' help next. Maybe do the hoses but its really cold this week.

    I see lots of talk about bleeding the ABS but all I saw for my OBD1 truck was bleeding the dumpoff valve and I don't have a bleeder screw, I will be loosening the brake line at the dumpoff. I have not seen an automatic bleed using a scanner for OBD1 trucks.

    For the tires:

    Stock 225 x .75 = 168.75 (29.3" diameter)

    Current 245 x .85 = 208.25 (32.4" diameter)

    I think the different tire sizes calc. out to a 10.5% increase in diameter. I'm not sure what the heavier tire does to braking so I want at least 10% more braking power.


    I found the following info recently and had not seen it yet in my research probably due to lots of old/bad links:

    https://www.aa1car.com/library/abs_kelseyhayes_rwal.htm
     
    Last edited: Jan 14, 2019 at 11:03 PM
  7. arrg

    arrg I'm Awesome

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    I went through all this with my 1988 C1500. I've tried a bunch of different things to get decent stopping power. Here's what I tried and what worked/didn't work:

    1. The original master was blown when I got the truck so I replaced it. There are two different size master cylinders for 88-91 trucks. You have a standard cab, so you're right you have the cheap tiny brakes and you need the smaller master cylinder. I got the bigger master cylinder and my brakes sucked.

    2. I took one look at the tiny booster and decided that a lack of brake boost must be the problem. I had a 1993 C1500 years before and the brakes worked fine on that. It had a bigger booster, so I replaced the booster on my 1988 with the bigger booster. It helped, but the brakes still sucked.

    3. I read on this forum about how people had swapped in the NBS master cylinder for improved braking. So, in went a brand new NBS master cylinder. I got a rock hard pedal, but the brakes sucked even more than before. Don't try this. The combination of the small D368 calipers with the NBS master will give dangerously little brake force at the front wheels. I could stand on the brakes hard enough to lock up the rear wheels and engage the RWAL on dry pavement but the front brakes just weren't doing much.

    4. Swapped in better brake pads in the front. It had cheap autozone pads before and then I swapped in Centric Posi-quiet semi metallics. This brought my brakes back to about step 2.

    5. After going this far and still having crummy brakes, I learned that the master cylinder changes I had made were making things worse. Going to the bigger master cylinder in step one reduced the pressure to brakes making them less effective. Swapping in the NBS master (it's even bigger) in step 3 made my brakes even less effective. The bigger master cylinder moves more fluid volume (this gives you the firmer pedal) but you have to input more force at the pedal to get the same force at the brakes as you had with the smaller master.

    6. At this point, my brakes still sucked, but I knew a little more about how brake systems worked. I had two options in my mind to fix the brakes. Either install a smaller master cylinder, or bigger front calipers. I decided to install the 8600 GVW (D370) calipers and keep the NBS master. My thought was if the calipers didn't get my brakes to where I wanted them to be, I could still swap in a smaller master for more brake force. The 8600 GVW calipers with the NBS master finally gave me the brake force I was looking for. I can lock up the fronts at will and the RWAL doesn't engage before the fronts lock up anymore.

    Disclaimer: I don't know if the 8600 GVW calipers will work on a 4WD with the tiny brakes. Hopefully someone else can chime in on that one.
     
  8. gordonm1

    gordonm1 Newbie

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    The NBS master and the bigger booster each make using the existing brake lines a bit more difficult to hook up. Each new part moves the connections further from the original location. I would try the calipers before the NBS. It sounds like it will be fine to replace the booster and the bigger master may not be needed.

    The link I provided discusses two causes for the soft pedal, a bad hydraulic unit or one that needs to be bled. I'm going to try bleeding first. My master is new rebuilt and it bench bled ok so if bleeding the dump off valve does not help I would bypass the RWAL/dumpoff valve.

    One question for the bypass is does this eliminate the safety feature of keeping fluid flow to the front or rear brakes if fluid is lost in one section front or rear? Maybe that is done with the proportioning valve?

    Another question is how is the electrical connector on top of the dumpoff valve removed to get to the pin under it? I can't find my manual or videos of this operation. It may be that the valve is reset when the motor is started and I won't need to reset it manually.


    My brakes make me want to say " AAARGH"
     

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