There was no boasting about either Lake Herman Road or Blue Rock Springs. The killer simply provided details and 'evidence' he was the responsible.
"This is the murderer of the 2 teenagers last Christmass at Lake Herman + the girl on the 4th of July near the golf course in Vallejo
To prove I killed them I shall state some facts which only I + the police know."
Christmass.  Brand name of ammo Super X  10 shots were fired  the boy was on his back with his feet to the car  the girl was on her right side feet to the west.
4th July.  girl was wearing paterned slacks  The boy was also shot in the knee  Brand name of ammo was western.
The decrypted 408 cipher contained a rambling message, but made no direct reference to either of the first three murders. The details he did provide in the three letters, were only given as evidence that the author was indeed the killer. No facts about how each crime unfolded and how he terrorized the victims was given by the Zodiac Killer.
This probably would have been the end of it regarding the Benicia and Vallejo murders, but for Vallejo Police Chief Jack E Stiltz, who still expressed doubts regarding the author's claims. He specifically asked for the killer to contact the newspapers or police again. Chief Jack E Stiltz was less than satisfied that the letter writer and killer were one and the same, as detailed in an extract from the Saturday August 2nd 1969 San Francisco Chronicle. He requested the writer to send a second letter "with more facts to prove it." Another Zodiac article featured the following day on Sunday August 3rd 1969 in the San Francisco Examiner. Jack E Stiltz was "still not convinced the letters and codes were written by the actual killer.' The police chief 'urged the writer to send more letters, with more facts to prove his connections to the crimes."
The response by the Zodiac was swift, when the following day the San Francisco Examiner received the 'Debut of Zodiac' letter giving the police chief what he had asked for: "In answer to your asking for more details about the good times I have had in Vallejo, I shall be very happy to supply even more material." Without the prompt by Jack E Stiltz it is extremely likely this communication would never have been composed and sent to the newspapers. Our knowledge of the 'pencil flashlight', the supposed negro eyewitness or his recollection of the Mageau shooting would never have been known. In other words, he didn't willingly brag about the crimes, he was prompted into a response by the utterances of the Vallejo police chief.
But for Jack E Stiltz, the Zodiac Killer may not have written from July 31st 1969 until October 13th 1969, when the Paul Stine letter arrived. In two and a half months the killer had murdered five people and attempted to murder two people, yet during this time period he would have likely written only two sets of communications. Not much of a braggart.
Therefore, the fact the killer failed to write extensively about his Lake Berryessa exploits was not particularly out of character.
His murder of Paul Stine on October 11th 1969 was followed by a relatively short letter. It declared "I am the murderer of the taxi driver over by Washington St + Maple St last night, to prove this here is a blood stained piece of his shirt. I am the same man who did in the people in the north bay area."
He was once again supplying evidence he was the killer, just like the July 31st 1969 letters, but added no details whatsoever about the actual murder of the taxicab driver or his exploits in the taxicab.
He added a brief description about his venture into the park: "The S.F. Police could have caught me last night if they had searched the park properly instead of holding road races with their motorcicles seeing who could make the most noise. The car drivers should have just parked their cars and sat there quietly waiting for me to come out of cover." The inability of police to capture him was highlighted, but again, there was no maniacal boasting about the murder or extensive details regarding it. Then came the prompting once more. This time by Chief of Inspectors Martin Lee.
Chief Martin Lee, speaking to the San Francisco Chronicle stated "His boast of being in the area we were searching while we were searching, it is a lie. We had the whole area flooded with lights. We had seven police dogs and a large number of patrolmen searching the area tree by tree and bush by bush. The dogs are the best in the country. A mouse couldn't have escaped our attention." link. He added "The fact Zodiac failed to mention the dogs and floodlights proves he wasn't anywhere in the vicinity." This last statement is patently flawed logic, because absence of evidence is not evidence of absence. This, coupled with the police stating Zodiac made mistakes, forced Zodiac into a response outlining details about the crime. Whether this was a cunning ploy by police to rile the Zodiac Killer into further communication and possibly trip himself up, or whether they really believed Zodiac never entered the park, can only be speculated. But it worked.
However, not once did the Zodiac Killer brag or boast about the callous murder of Paul Stine. He reacted to the police claims, by retorting "2 cops pulled a goof, hey pig doesnt it rile you up to have your noze rubed in your booboos, the police don't have much to work on, as of yet I have left no fingerprints behind me contrary to what the police say, and I was leaving fake clews for the police to run all over town with."
The rest of his communication was reserved for the bomb he was allegedly constructing. The Zodiac Killer was full of threats, but so far hadn't willingly boasted about the very nature and details of his chilling crimes.
The final line of the 'Bus Bomb' letter is testimony to the agenda of the Zodiac Killer up to this point.
"To prove that I am the Zodiac, Ask the Vallejo cop about my electric gun sight which I used to start my collecting of slaves." The Zodiac Killer was referring to the Vallejo cop Jack E Stiltz, who prompted the 'Debut of Zodiac' letter, asking for more details, and Zodiac did respond- informing Jack E Stiltz about his 'pencil flashlight' or electric gun sight. The 'Bus Bomb' letter was in direct response to Chief Martin Lee, to whom, he was trying to prove he did venture into the park and didn't leave any evidence at the crime scene. However, what is evidently lacking in this communication, is the lack of glorification in the murder or detailing of the actual crime itself.
The Zodiac Killer was undoubtedly a ruthless and merciless killer, but as of November 9th 1969, the closest the Zodiac Killer came to boasting about the actual murders and attempted murders thus far, was when asked to do so.
He replied to Jack E Stiltz "I fired the first shot at his head, he leaped backwards at the same time, thus spoiling my aim. He ended up on the back seat then the floor in back thashing out very violently with his legs; that's how I shot him in the knee" and "All I had to do was spray them as if it was a water hose."
The 'Bus Bomb' letter may still have been mailed, but that is probably all it would have contained- details of his bus bomb and the threat of more murders, and likely little or no reference to the murder he had already committed. Had he wanted to provide copious material about the Paul Stine murder, he had every opportunity when he mailed the October 13th 1969 letter, but he refrained from doing so. The premise of the Zodiac Killer as somebody who enjoyed boasting about his crimes is not apparent in his writings, unless pushed. The threats however, would still continue.
The Zodiac Killer thought he had provided all the evidence required to prove he was the Lake Berryessa perpetrator, by writing on the car door of Bryan Hartnell's white Karmann Ghia and making the telephone call, yet it wasn't enough for some. Despite this being his most audacious crime to date, he again remained deafeningly silent on the specifics of the attack. Bryan Hartnell's survival and testimony of the crime, would inadvertently give the Zodiac Killer all the newspaper coverage of the actual crime he needed, if this was ever a requirement of the Zodiac Killer.
Every correspondence to date, other than the extra details he was prompted into supplying, had one thing in common.
July 31st 1969: "If you do not print this cipher by the afternoon of Fry.1st of Aug 69, I will go on a kill ram-Page Fry. night. I will cruse around all weekend killing lone people in the night then move on to kill again, until I end up with a dozen people over the weekend."
October 13th 1969: "School children make nice targets, I think I shall wipe out a school bus some morning. Just shoot out the front tire + then pick off the kiddies as they come bouncing out."
November 8th 1969: "This is the Zodiac speaking. I though you would nead a good laugh before you hear the bad news. You won't get the news for a while yet. Could you print this new cipher in your frunt page ? I get aufully lonely when I am ignored, so lonely I could do my Thing."
November 9th 1969: "What you do not know is whether the death machine is at the sight or whether it is being stored in my basement for future use. I think you do not have the manpower to stop this one by continually searching the road sides looking for this thing. + it wont do to re roat + re schedule the busses because the bomb can be adapted to new conditions. Have fun!! By the way it could be rather messy if you try to bluff me. PS. Be shure to print the part I marked out on page 3 or I shall do my thing."
It was the threat of the crime itself, not the basking in the crime that had passed. The Zodiac Killer was prompted into ridiculing the police and supplying them with "more details" about the crimes, but terror was the 'thing' he enjoyed most.