jaguargt Posted January 9, 2018 Share Posted January 9, 2018 Hi guys, I'm having a few issues with my brakes: is extermely hard to stop my car, the pedal feels like a rock after about an inch of travel. I have a 75 280z with Lt1 and a 503 cam, 280zx 10" booster (new), 1" wilwood MC (new), front stock disc brakes and rear drum brakes with all new hardware, and 9 inch of vacuum at the booster port.. Do you think the 9 inch of vacuum are too low or could be any other thing to check? or maybe having a 10 inch booster and 1 inch MC could be compensated installing toyota 4 pistons caliper? Thank you in advance, Selvin Quote Link to comment Share on other sites More sharing options...
Chickenman Posted January 9, 2018 Share Posted January 9, 2018 (edited) 9in/hg is probably too low. You need about 12 in/hg minimum. A larger Booster or an Auxiliary Brake Booster Tank will help. You can also increase Idle Manifold vacuum, significantly by Increasing timing advance at idle. Cams with more overlap require more ignition advance at idle. Are you carburated or Fuel injection. You need to run a digressive Vacuum advance curve ( Carb ) or Load curve ( EFI ). With a carb you need a Vacuum advance that has adjustable spring tension and curve . You set the spring tension very weak, mso that full Vacuum advance is all in by 8 to 10 in/hg. Set the stroke length to give you about 10 degrees of total Vacuum advance. Then connect the Vacuum pot to Manifold Vacuum, so you have full Vacuum at idle. Set your initial timing to about 14 to 16 degrees BTDC at 700 rpm, with the Vacuum advance hose disconnected. Then connect the Vacuum advance hose to give a total of 24 to 26 degrees . Then adjust idle to about 1,000 RPM. You should find that this gives a considerable increase in idle Vacuum over Ported Vacuum, or no Vacuum advance at all. The only difference between Ported Vacuum and Manifold Vacuum, is that Ported source does not pull any Vacuum at idle. Manifold sourced vacuum does. As soon as you open the throttle Blade slightly, they both behave the same. But the extra 10 degrees of Timing with the Manifold Vacuum really helps cammed engines idle better, use less throttle blade opening to achieve a set idle speed and thus pull more Vacuum at idle. Stand alone EFI is a bit easier to do. You just play with keystrokes and builds the extra timing in by a Programmable curve instead of messing with Vacuum pot spring tensions and stop limiters. Note. It is very important when using a regular distributor with Mechanical weights, that your advance curve does not start too low. I always set mine so it started around 1,200 rpm with an idle speed of 900 to 1,000 RPM. Having too weak of advance springs will cause the timing to " bounce " at idle. Will cause erratic idle and erratic Vacuum readings. Edited January 9, 2018 by Chickenman Quote Link to comment Share on other sites More sharing options...
Chickenman Posted January 9, 2018 Share Posted January 9, 2018 You didn't mention what Generation of LT1 you have. Early style Gen 1 (1970- 1972 ) or late style Gen 2 ( 1992 - 1997 ) or Gen V ( 2013 - Current ) . The later style have smaller combustion chambers and require a bit less initial ( mechanical ) timing than the Gen 1. Fuel quality also plays a part of course. So you do have to experiment a bit. Quote Link to comment Share on other sites More sharing options...
jaguargt Posted January 10, 2018 Author Share Posted January 10, 2018 Thanks a lot for the answer. My engine is a LT1 95, EFI, with stock PCM, I already have Tunner Cat to work on the tunning. I think that I could try advancing the time and I will change the spark plugs too, I was using stock ones and I read that the recommended for the 503 Cam are TR55 NGK. For now the plan would be: 1. Bleed the system, just in case.2. Re-adjust the rear brakes. 3. Check pedal free travel (rod lenght) 4. Apply vacuum to the booster with an external pump, just to try if there is an improvement, if there is, modify the ignition time with tuner cat. (or maybe a dual diaphragm booster?) 5. Now I think I'm clear that updating to 1" master cylinder without changing the stock components was an error, so, when my budget allow me, for start swap to toyota calipers, I already have the calipers for vented rotors, so I need rebuild the calipers, buy the rotors and spacers. Of course, any other ideas are welcome. Thanks again, Quote Link to comment Share on other sites More sharing options...
NewZed Posted January 11, 2018 Share Posted January 11, 2018 On 1/9/2018 at 7:19 AM, jaguargt said: Hi guys, I'm having a few issues with my brakes: is extermely hard to stop my car, the pedal feels like a rock after about an inch of travel. Do you mean that the brakes don't slow the car? Or that the brakes are difficult to control? The rock-hard pedal is typical of a missing reaction disc in the booster. Measure vacuum at the end of the booster supply hose. Maybe you have the check valve installed backward. The MC cylinder size won't affect pedal feel as much as pedal travel. Quote Link to comment Share on other sites More sharing options...
Chickenman Posted January 11, 2018 Share Posted January 11, 2018 You should be able to " Free Rev " the engine a few times and build up the Vacuum in the Booster. On the " Over run " Vacuum will build higher in the Booster. Wing it hard to about 4,000 a few times. Let it return all the way to idle. Repeat about 3 or 4 times. Then press brake pedal. If brake pedal is easier to press, then you know the issue is just low idle Vacuum. If the pedal is still Rock Hard then you may have a problem with the Booster check valve ( Backwards or leaking ) or the reaction disc fell out when you changed the Brake M/Cyl Quote Link to comment Share on other sites More sharing options...
jaguargt Posted January 12, 2018 Author Share Posted January 12, 2018 Thanks good advices, I have chedked the end of the booster supply hose, 9inch of vacuum. I will try with Chickenman advice about revving the engine, good tip. The booster is new, could be a chance that the reaction disc is out of place, I will check that too. Thanks again, I will keep you posted, Selvin Quote Link to comment Share on other sites More sharing options...
Miles Posted January 12, 2018 Share Posted January 12, 2018 With the engine off, place firm pressure on the brake pedal and hold that pressure. While holding pressure on the brake pedal start the car and post what your foot does. Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 5, 2018 Share Posted February 5, 2018 With an aggressive cam, you're likely always going to be on the low side for acceptable vacuum for the booster. A better solution for this would be to get an electric brake booster vacuum pump from any number of diesel cars out there. Wire it up with a hobbs switch to regulate the vaccum and you're good to go. Here's the pump that I use: https://www.ebay.com/itm/2007-2008-2009-AUDI-Q7-4L-ELECTRIC-BRAKE-BOOSTER-VACUUM-PUMP/152867717124?fits=Make%3AVolkswagen&hash=item2397a04004:g:fpoAAOSw2LZaXTPT&vxp=mtr Quote Link to comment Share on other sites More sharing options...
munters Posted February 8, 2018 Share Posted February 8, 2018 Onether point. The bigger the mc the more force needed for the same pressure. Why did you go with a 1inch mc and stock brakes? Quote Link to comment Share on other sites More sharing options...
Miles Posted February 12, 2018 Share Posted February 12, 2018 On 02/07/2018 at 11:28 PM, munters said: Onether point. The bigger the mc the more force needed for the same pressure. Why did you go with a 1inch mc and stock brakes? WIth stock brakes, a 1 in. MC is expected to make the pedal hard to push. Replace the 1 in. MC with the stock 7/8 in. MC and your brakes will be fine. 1 Quote Link to comment Share on other sites More sharing options...
nmehdikh Posted February 12, 2018 Share Posted February 12, 2018 On 2/4/2018 at 9:16 PM, TimZ said: With an aggressive cam, you're likely always going to be on the low side for acceptable vacuum for the booster. A better solution for this would be to get an electric brake booster vacuum pump from any number of diesel cars out there. Wire it up with a hobbs switch to regulate the vaccum and you're good to go. Here's the pump that I use: https://www.ebay.com/itm/2007-2008-2009-AUDI-Q7-4L-ELECTRIC-BRAKE-BOOSTER-VACUUM-PUMP/152867717124?fits=Make%3AVolkswagen&hash=item2397a04004:g:fpoAAOSw2LZaXTPT&vxp=mtr Did you also run a vacuum reservoir? Or are you just running the pump to the booster? Quote Link to comment Share on other sites More sharing options...
TimZ Posted February 15, 2018 Share Posted February 15, 2018 On 2/12/2018 at 1:01 PM, nmehdikh said: Did you also run a vacuum reservoir? Or are you just running the pump to the booster? I just run it straight to the booster, but if you can find room for a reservoir, it certainly wouldn't hurt. Quote Link to comment Share on other sites More sharing options...
munters Posted February 15, 2018 Share Posted February 15, 2018 I would fix the cause and not putt a band-aid. Change the MC. I installed a bigger clutch MC and changed it finally because of a to hard pedal for my wife and addressed the cause(wrong throw out bearing) learned my lesson... Quote Link to comment Share on other sites More sharing options...
Kevinbn7 Posted February 20, 2018 Share Posted February 20, 2018 I just dealt with the same condition. Had a 1 inch wilwood that had a rock hard pedal and would not stop the car. Changed to the 7/8 and the brakes feel correct. I can actually get to the threshold of lock up and the car stops like it should. Bigger is not better in this case. Quote Link to comment Share on other sites More sharing options...
jaguargt Posted February 28, 2018 Author Share Posted February 28, 2018 Thanks for all the inputs, I Installed a larger MC because the plans was to install the larger Toyota calipers and ventilated rotors, but I have been a little short on budget, I just ordered the spacers and the rotors, and I'm waiting for them to arrive (I live in Guatemala so that will take a few weeks ). And just as aditional info, I installed an electrical vacuum pump, 20 inch of vacuum, and the feeling was the same, so I opted to finally swap to the toyota calipers. I will keep you posted, Thanks guys, 1 Quote Link to comment Share on other sites More sharing options...
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